2 sticks (1 cup) unsalted butter, room temperature (1 cube of 227g unsalted butter)
2 1/3 cups sugar (502.5g). Not too sweet, try 450g
5 large egg whites, room temperature (White)
2 teaspoons pure vanilla extract
1 1/2 cups milk, room temperature
Red, orange, yellow, green, blue, and purple gel food coloring
Instructions
Preheat oven to 350 degrees. Brush six 9-inch-round cake pans (or
as many 9-inch cake pans as you have, reusing them as necessary)
with shortening. Line bottom of each cake pan with parchment
paper; brush again and set aside.
In a large bowl, whisk together flour, baking powder and salt; set
aside. In the bowl of an electric mixer fitted with the paddle
attachment, cream together butter and sugar. Slowly add egg
whites and mix until well combined. Add vanilla and mix until fully
incorporated. Add flour mixture and milk in two alternating
additions, beginning with the flour and ending with the milk. Mix
until well combined.
Tips- Use the butter whisk first then switch mixer device for even
mix.
Divide batter evenly between six medium bowls. Add enough of
each color of food coloring to each bowl, whisking, until desired
shade is reached. Transfer each color to an individual cake pan.
Transfer to oven and bake until a cake tester inserted into the
center of each cake comes out clean, about 15 minutes (working in
batches if necessary).
Remove cakes from oven and transfer to a wire rack; let cool for 10
minutes. Invert cakes onto a wire rack; re-invert and let cool
completely.
Using a serrated knife, trim tops of cakes to make level. Place four
strips of parchment paper around perimeter of a serving plate or
lazy Susan. Place the purple layer on the cake plate. Spread a scant
1 cup buttercream filling over the first layer with a small offset
spatula so it extends just beyond edges. Repeat process with blue,
green, yellow, and orange layers.
Place the remaining red layer on top, bottom-side up. Gently sweep
away any loose crumbs with a pastry brush. Using an offset spatula,
cover the top and sides with a thin layer of frosting (also use any of
the excess frosting visible between the layers). Refrigerate until set,
about 30 minutes.
Using an offset spatula, cover cake again with remaining frosting.
