Transfer bananas to fine-mesh strainer, or a strainer lined with a single layer of cheese cloth and set over bowl to drain, stir occasionally. Leave it to drain about 15 minutes. You should collect about ¾ to 1 cup liquid (180-240ml).
Move the liquid to a medium saucepan (the idea is to have a shallow layer of the banana juice so it reduces quickly) and plop the bananas into a bowl, if refrigerating until ready to make the muffins (i.e overnight) - pick a bowl that's right sized for the bananas, if moving on to make muffins right away, this should be a medium mixing bowl.
Cook banana juice over medium-high heat until reduced to ⅓ cup, 8 to 10 minutes. It'll darken up, and when you stir, you'll see the bottom of the pan, it'll take a moment for the liquid to fill back in. If needed, measure from time to time. Pour liquid into the bowl with the bananas and mash with potato masher until mostly smooth.
If you're setting this aside to make muffins later: cover the bowl, ideally using cling film/plastic wrap in full contact with the bananas, but a cover is a cover, they will brown up overnight. I don't know how long you can let it go, I wouldn't wait more than a day to make the muffins. Take them out of the fridge and bring back to room temperature, then resume with next step when ready.
Set oven rack to upper-middle or middle position and heat oven to 400F/200C/180C fan. Grease 12-cup muffin tin.
Toast walnuts and sunflower seeds in dry 12-inch skillet over medium heat for about 5 minutes, stirring occasionally before adding the oats and toasting for an additional 3-5 minutes until golden and fragrant. Remove from heat to cool - if you use cast iron, move the mix to a separate bowl to cool or it will continue to toast in the pan.
Whisk together butter, eggs, and vanilla, add in the banana mixture (or, if doing this in one swoop, whisk the others into the bananas).
In large bowl, whisk the dry ingredients: flour, baking powder, baking soda, and salt together. Stir in banana mixture with a sturdy spoon (I broke a spatula!) until combined. Add milk if needed to loosen up the batter. Fold in the toasted mixture, the walnuts, oats, and sunflower seeds.
Divide batter evenly among muffin cups, for me, it filled them completely. Bake until toothpick inserted in center of muffins comes out clean, about 18 to 22 minutes, rotating muffin tin halfway through baking.
Let muffins cool in tin for 10 minutes, then transfer to wire rack and let cool for 20 minutes before serving.
Once cooled, melt 20g of 70% dark chocolate in a heat proof bowl set over simmering water. Spoon the chocolate over the cooled muffins, let the chocolate set and enjoy.