1cupunsalted butter, at room temperature (226 grams)
2cupsgranulated sugar(400 grams)
1cupwhole milk(237 ml)
2teaspoonsvanilla
5large egg whites
Red gel food coloring
Blue gel food coloring
For the vanilla buttercream:
1 ½cupunsalted butter, at room temperature (340 grams)
6cupspowdered sugar, sifted (1 kg 360 grams)
Pinchof salt
1tablespoonvanilla extract
¼cup+ 2 tablespoons heavy whipping cream(90 ml)
Sprinkles, if desired
Instructions
To make the cake:
Preheat oven to 350°F. Butter and flour 8-inch round pans and line with parchment paper. Since you'll need 5 layers total, use as many 8-inch pans as you have and bake in batches if necessary.
In a medium bowl, combine the cake flour, salt and baking powder. In a separate bowl combine the milk, vanilla and egg whites. Set both aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar on medium high speed until light and fluffy, about 3-4 minutes. Scrape bowl down with a spatula a few times as needed. Add the dry ingredients in 3 additions, alternating with the wet ingredients, and ending with the dry. Mix only until incorporated.
Divide batter between 5 small bowls. For even layers, weigh the batter and divide evenly. Leave one bowl of batter as is and using the gel food coloring, tint the remaining bowls as such: 1) bright red, 2) light red, 3) bright blue, 4) light blue.
Pour batters into individual prepared cake pans and bake until centers spring back when touched, about 14-16 minutes. Bake in batches as needed to finish all 5 layers. Let cake layers cool in the pan 5 minutes, then transfer to a wire rack to cool completely.
To make the buttercream:
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter, powdered sugar, salt, vanilla, and whipping cream on medium speed until moistened and uniform. Increase speed to high and beat until light and creamy, about 4-5 minutes.
!To assemble the layer cake:
Place the bright blue cake layer on a display plate or cake stand. Cover with ½ cup of the buttercream. Top with the light blue cake layer. Repeat so that the cake layers are in the following order: bright blue, light blue, white, light red, bright red.
Cover the layer cake with the remaining frosting and decorate as desired. Chill for at least 1 hour before serving to neat and clean slicing. Store leftovers in the fridge for up to 5 days.