To prepare the cake:

1. Preheat the oven to 350 degrees and prep a rimmed half sheet pan (18”x13”) by lining it with a sheet of parchment paper and lightly greasing the sides.
2. In the clean bowl of a stand mixer or a large mixing bowl, beat the egg whites on medium-high speed until stiff peaks form. Set aside in another bowl while you continue prepping the cake.
3. In the same stand mixer bowl you whipped the egg whites in, cream the butter and sugar for 2 minutes. Scrape the sides of the bowl. Combine 3 cups of the flour, baking powder, and salt and add half of this mixture to the butter mixture. Stir on low to almost combine and then add half of the milk and the vanilla. Stir to combine and then add the remaining half of the dry ingredients followed by the remaining half of the milk. Use a rubber spatula to gently fold in the egg whites being careful not to overwork the batter. Smooth the batter into your prepared pan and bake until a toothpick inserted comes out clean, about 19-20 minutes. Be careful to not overbake! The top will be golden and set when the cake is done. Set aside to cool completely- you can expedite this process by popping it in the freezer after cooling on the counter for a bit. Once cool, use a 2” round biscuit or cookie cutter to trim out circles of cake. You should be able to get about 23 circles out of the sheet cake which will make 7 cakes plus 2 extra layers for backup. Feel free to pop the layers on a clean baking sheet lined with parchment paper and freeze briefly while you make the frosting. This will make assembly a little easier.
4. To prepare the frosting, whip the butter on medium speed using the paddle attachment on your mixer. Continue for about 3 minutes, scraping the bowl as necessary, until pale and smooth. Add half of the sugar and stir to combine. Add the remaining sugar, the vanilla, and a couple of tablespoons of milk or water to help it all come together. Stir and add water or milk a tablespoon at a time until the frosting is smooth. Consistency is important here- make sure the frosting isn’t so stiff that it will pull on and rip the cake layers while assembling, but it also needs to be thick enough so that it won’t just squish out the sides. A good consistency frosting will plop off your rubber spatula but will chill to firm. Pop it in the fridge if it gets too warm.

To assemble the cakes:

1. Each mini cake will have 3 layers. Mix up your colors as you please. For the colors pictures here, I used Americolor Food Gels. The pinkish one is a 3 parts red, 2 parts blue, and ½ part brown. I recommend adding the color a tiny bit at a time to make sure you don’t mix a color you’re not fond of! The blueish greenish cake is 3 parts blue with ½ part brown. The orange-ish cake is 3 parts deep pink with ½ part brown.
2. When ready to assemble, smear a little frosting on your cake board or serving plate and stack the first layer. Add about 2 tablespoons of frosting on top and use an offset spatula to smooth. Stack a second cake layer and repeat this process twice to finish with a third cake layer. Spread a thin crumb coat of frosting on the cake and pop it into the fridge or freezer to set up. I placed a cooling rack with a sheet of parchment on it in my freezer so that I could quickly chill the cakes while I was making them. I found keeping the cakes cold while assembling helped make the process simple.
3. Once your crumb coat has set up, finish frosting the cake with the remaining frosting and decorate as desired! For my cakes, I use a round dollop of white frosting that I smudged with the end of my offset spatula. White large and small nonpareils and clear sprinkles finished them up! Share with someone you love and enjoy!