Smith Island Rainbow Cake

Smith Island Rainbow Cake

The Recipe

Rainbow cake wedge

Rainbow Smith Island Cake

This is a Smith Island Cake in the thickness of its layers only, there will be someone who will point out that this is not a true Smith Island Cake, and they are 100% correct. This is a franken cake. It's 3 recipies deliciously squashed together. A rainbow cake, baked in Smith Island Cake inspired thin layers and iced with an icing from a Mary Berry recipe. We have both the US and the UK represented here in cake. It doesn't have to be a rainbow – what about a halloween themed cake in orange, green, and purple layers, maybe with a dark chocolate icing? Or a gradient of your favorite color? Flag, school, or company logo colors? but yeah, here I'm going to be pretty rainbow specific.
Course Dessert
Cuisine American, Maryland


Cake Batter

  • 1 cup (240 ml) whole milk room temperature
  • 6 large egg whites room temperature
  • 1 teaspoon vanilla extract
  • 250 g (9 ounces/ 2¼ cups) cake flour in the UK: 35g corn starch and 215g plain flour
  • 340 g (12¼ ounces/ 1¾ cups) sugar
  • 4 teaspoons baking powder
  • 1 teaspoons salt
  • 170 g (16 oz/ 12 Tablespoons/ 1.5 sticks) unsalted butter, cut into 12 pieces and softened
  • Gel food dye red, orange, yellow, green, blue, purple, and pink

White Chocolate Icing

  • 300 g (10½ oz) White chocolate either chips or the thin melting discs or a couple chopped up bars
  • 150 g (5 oz /about 10½ TBSP) butter softened
  • 225 g (7½ oz) cream cheese
  • 600 g (21 oz/ 5¼ cup) icing (powdered) sugar sifted to remove lumps
  • 3 tsp vanilla extract


Make the cake

  • Set the oven rack to the middle position and preheat your oven to 350°F/180°C/160°C fan. Grease two 8-inch round cake pans, set a circle of parchment in the bottom and grease it and a bit of the sides again for extra measure and tap flour in the pan to cover the base and at least ½ inch up the sides.
  • Whisk together milk, egg whites, and vanilla in an easy to pour from container (I like a medium sized bowl with a pour spout). Using stand mixer with the paddle attachment, mix flour, sugar, baking powder, and salt on low speed until they are combined.
  • Add 12 tablespoons butter, 1 piece at a time, and mix for about a minute until the largest sized pieces are about pea-sized. Add half of the milky egg white mix and increase speed to medium-high. Beat the batter until it's light and fluffy, this will take another 1 minute or so. Slow the mixer down to medium-low, add remaining milk and egg mixture, and beat until incorporated, about 30 seconds. The batter will look curdled, this is good. Give it a stir by hand for good measure to check the batter's consistency.
  • Measure ¾ cup (180ml) of batter into two bowls and add and whisk in gel food dye to reach the colour desired. You'll see white flecks in the batter, the final colour of this layer will be the color you see BETWEEN the white flecks, so watch that and don't worry about the white bits.
    dyed cake batter
  • Pour each batch of coloured batter into its own prepared cake pan. Tilt the pan to cover the whole bottom evenly with batter, give it a wee shoogle (shake) and tap it, level, on the counter once or twice to remove air bubbles. Pop the cake pans in the oven and bake, swapping and rotating them halfway through, until a toothpick inserted in center comes out clean (it's really thin, so go at an angle) or the top has the beginnings of a light golden color, about 10-15 minutes.
  • Leave the layers to cool in their pans on a cooling rack for 10 minutes. Slip a knife around the edge of each the cake layer and remove them from the pans, peel back the parchment and discard (I used a reusable parchment disc- in that case – wash and dry it for the next layer!). Let the layers cool completely on a cooling rack, they're nice and thin so this won't take long.
  • Repeat steps 4-6, measuring the batter, dying and baking it, with the remaining batter. You'll get 7 layers.

Prepare the construction icing… (⅔ of the batch)

  • The very first thing you should do here, before getting out any other ingredients is cut up the butter (100g of it for this batch) and set it out to soften, then move on to melting the chocolate.
  • Put 200g (4¾ oz) of the white chocolate in a bowl over a small pot of simmering water, the water should not touch the bowl and it should be just simmering, it'll heat up further once the bowl's been on it for a little bit and you don't want to overheat the chocolate. I like to use a glass bowl and a rubber spatula so I can watch below for boiling.
    If it starts to boil, take the bowl off, lower the heat and stir the chocolate while waiting for the water to cool back down to a simmer, then replace the bowl on top of the pot if further melting is required.
  • Stir the chocolate as it melts and remove the bowl from the heat while there are still visible lumps, keep stirring until the last of the lumps melt and the chocolate is smooth, you can move it back and forth over the hot water if the last of the lumps are a bit stubborn, set this aside to cool and thicken while you gather up the other ingredients and start making the icing.
  • NOW gather up and prep the other ingredients.
  • Using your mixer with a whisk attachment, whisk together 100g (3.5 oz) butter and 150g (5 oz) cream cheese until soft and fluffy, the colour should be uniform and there should be no lumps of butter. If you're seeing lumps of butter, it's not softened enough, you may need to let it sit for a while longer and come back to whisk further.
  • Add 200g (7 oz) of the icing sugar and whisk to fully combine.
  • Add 2 tsp vanilla and 200g (7 oz) icing sugar and whisk until fully or nearly fully combined.
  • Add your cool-to-warm melted white chocolate – if it's still hot, hold off for a bit, you don't want it further melting the butter. Whisk until it's fully combined. Sometimes it's smooth at this point, because it's still a little warm and not ready to be worked, sometimes it's fully combined, but looks a bit rough, almost like it breaks apart when the whisk passes rather than stretches, this because it's cooled down enough to start assembling the cake.
  • If needed, transfer to the fridge for 10-20 minutes until it's cooled and spreadable.
    If it's gone too cold, you can put the mixer bowl into a larger bowl of warm (not hot – just warm to the touch) water to help gently warm it back up.

Assemble the cake

  • Spread a fine layer of icing in the middle of your cake platter and set your first layer of cake (I suggest running the rainbow in reverse – so working from bottom to top: pink, purple, blue, green, yellow, orange, red). You can put a few bits of parchment under the edges of the cake covering the platter edges to protect it from stray icing. Spread a thin coat of icing over the pink layer and set the purple layer. Continue through all the layers covering each with enough icing to cover it. Cover the top layer and then work the icing down the sides to cover. Refridgerate about 15 minutes until the frosting is set, or set it aside at room temperature (I left it out for a couple hours).
    rainbow cake under construction

Make up the top coat of icing (the remaining ⅓ batch)

  • Repeat the icing process, with the remaining ingredients – here's the quick version:
  • Melt 100g (3½ oz) of white chocolate in a glass bowl over a sauce pan with just simmering water. Stir as it melts, remove from heat and keep stirring when there are still lumps in the chocolate, stir until smooth. Set aside to cool.
  • Whisk 50g (1¾ oz) butter and 75g (2.5oz) cream cheese until uniform and "fluffy" in your stand mixer with the whisk attachment.
  • Add 100g (3½ oz) icing sugar and whisk to combine.
  • Add 1 tsp vanilla extract and 100g (3½ oz) sugar and whisk to combine. If you want to dye this layer of icing, now's a good time, remembering that after the chocolate goes in, the color will be a little lighter, and once that happens, you can always go darker, but getting lighter means making more icing.
  • Add your now cool/warm melted white chocolate and whisk to fully combine, adjust colour if you're colouring this batch of icing. If it's spreadable now, you're good to go, if it's still a little too warm, you can cool it down to spreadable in about 10-15 minutes in the fridge.
  • Spread this last batch over the entire cake to your preferred finish and decorate as you like. Ours was an "Under the Sea" scene, so the icing was swirly like waves and we added decorations made of white fondant dyed with food colouring gel, and cut with cookie cutters.
    Under the sea rainbow cake


Birthday Cake/Life Balance:
This is a project. The first year I made this, taking meal breaks (I’m sure I didn’t cook dinner too but I might have), it took pretty much all day and well into the night which was ok for a Sunday party.
Doesn’t work for a Saturday with a full time job. 
You can freeze the layers, I froze them for 5 days, I’d probably stay close to a week. I put buttered parchment or reuseable parchment between the room temperature layers. One side of each layer is a little tacky, I put that side against the parchment and the bottom layer – smooth and not sticky – facing the cling film. I wrapped them in cling film in groups of 2-3 layers, and then built a comical little structure for freezing these layers in the freezer (shelves made of grocery store cardboard pizza circles and building blocks) that last bit might be overkill, you could probably put in separators and freeze them all in 1 bundle. I just didn’t want to risk the weight of upper layers squishing lower layers. 
They thawed beautifully and were easy to work with.
Cake flour:
In the UK, cake flour is not a thing. I have good results with Nigella Lawson’s substitute. By these maths (I went UK there!) to get 250g of cake flour, we’re combining:
  • 35g corn starch
  • 215g plain flour
Color tip: 
If you’re going to have pink and red, I suggest making the pink a bit paler, I love a bright vibrant color, but with my dyes, the pink and red layers came out very similar. Next time I’ll tone back the pink so it’s obviously a different color.
Since this makes 7 layers, and I work with 2 cake tins, I like to bake 1 layer first, this gives you a good idea of the baking time, it’s a good test layer, if something goes wrong, you lose 1 layer of cake, not 2 or 3.
If your oven fits more cake tins and you have them – bake more layers at a time, just be sure to rotate the tins and swap their positions in the oven at about the 6-7 minute mark so they bake evenly. 
Having 4 cake tins, and baking 2 at a time (or 6 tins, baking 3) is a big time saver, you can clean and prep the next layers while the first layers are baking and cooling in their tins. 
Getting Butter to Room Temperature in a hurry:
If your butter is stubborn and/or your kitchen runs a little cold and you have a microwave:
Set the microwave to 30% power and microwave the butter checking it every 5 seconds. Also, because it probably won’t do a full rotation, check butter in several spots on the plate and rotate the plate between 5 second runs. 
Our microwave resets to 100% after each timed cook, so I just set it to 30 seconds and pop the door open every 5. I seldom need to run more than 3 rounds.
Keyword cake, kids, layer cake, maryland, white chocolate

The Waffle

This is the first time I’ve made a layer cake entirely on my own, and I’m pretty proud of the fact that I went full Maryland on it. I’ve heard from a couple folks that the rainbow cake is a big hit, but there are cons to that pro, and they mostly relate to its height and the sheer volume of cake that results. Hearing all this I thought “there’s a simple solution – Smith Island Cake”. Lots of places and cultures have a “Smith Island Cake” – Ukraine and Lithuania both have Honey Cake, German Schichttorte, Croatia has Mađarica, heck, other states in the South Eastern United States have cakes that look just like Smith Island Cakes, but I was raised in Maryland and that’s my point of reference.

So, I took the America’s Test Kitchen Rainbow Cake recipe and winged it, baking it Smith Island style, I was mentally prepared to make 2 batches of the batter and have a ton of batter left over, I was going to make rainbow swirl cupcakes, but a single half batch of the batter as printed did the trick beautifully with a little more than what was needed to make an 8th layer. Most of the layers were great, the red layer baked a little thin in spots, it was my first layer and I didn’t quite get the knack for spreading the batter around the pan, but I think making 7 slightly thicker layers is the way to go, certainly with this batter, if I do this again. Seven may not be enough layers to fully qualify as a legit Smith Island Cake.

The Mary Berry white chocolate icing recipe says you really should use Belgian chocolate because of it’s fat content, but I’m here to tell you, we bought the most wrong stuff on the planet that wouldn’t work in a million years in this recipe, and I only found out at about 9pm the night before the birthday party, so I had no choice but to go to the (granted, M&S branded) petrol station convenience store nearby and buy literally the cheapest white chocolate you can buy here in the UK and it worked a treat. 4 year olds and their parents were happy. Tasted great and held 8 layers of cake together, that’s what icing is made to do.

If I ever make this again, I’m investing in 2 more 8 inch/20 cm cake tins. That way I can have 2 cakes cooling on the rack in the tins while 2 are baking, that would shave a fair amount of time off the baking time. I’m not confident 3 cake tins would fit in our oven, or I would consider doing 3 at a time, with 2 sets of tins.

It’s a sturdy cake, with a bit of heft, it may have been the bright colours, or the exhaustion from a late night up baking and decorating, but I swear it tasted a bit like fruity pebbles.