Grease a 9½-by-13-inch shallow baking sheet, or flat bottomed baking dish with cooking spray, if needed, smooth it out with a paper towel. Sprinkle icing sugar in the pan and thoroughly coat, tap and dump out excess. Prepare decorating sugar if like me you don't just keep fancy sugar around.
Pour 160 ml cold water in your mixer bowl and sprinkle the gelatin over it. Let stand for at least 5 minutes while you move on to cooking the sugar syrup.
Get out a pastry brush and a small bowl of water, have it near the stove. It's good to have your mixer close to the stove here too.
Heat granulated sugar and 118ml (½ US cup) room-temperature water in a medium saucepan over medium-high heat, stir until sugar dissolves and the liquid is smooth. Wash down sides of pan with a wet pastry brush and attach your candy thermometer to the pot. Stop stirring and watch thermometer, nae stirring from here til it's off the heat.
Cook until syrup reaches the soft ball stage, 238 degrees Fahrenheit on your candy thermometer. Remove from heat immediately.
Slowly pour syrup into your mixer bowl, it will bubble up, so be careful here. Start the mixer on a slow speed. At this point you're just combining the ingredients, dissolving the gelatine and starting to cool the syrup. Keep whisking at this low speed for a few minutes. Turn the mixer up to medium high and then high speed (maybe not your top speed, but close) until soft peaks form. It's ready when the marshmallow whip makes that classic peep beak when the peak flops over. The recipe I read said 8 to 10 minutes but honestly, it was more like twice that.
When the bowl is cooled down and the marshmallow has gone perfectly white and you think it's nearly done, add the vanilla and whisk for another minute or two. If you're not to soft peaks yet, start up the mixer again and keep going.
Pour the marshmallow fluff into prepared baking sheet and smooth the top using an offset spatula. Sprinkle with sanding sugar, and smooth the sugar out using the cleaned offset spatula, shoogle the tray about too, tilting and tapping - like you're coating the bottom with the icing sugar. Let stand for at least 1 hour to set. This can be done the day before you plan to cut out your peeps.