• Preheat the oven to 350 degrees fahrenheit. Spray 6 8-inch cake pans with cooking spray and line with parchment rounds. If you don’t have 6 cake pans, use what you have and bake in batches. I baked two cakes at a time.
• In a large bowl whisk together the flour, sugar, baking powder, and baking soda. Add in the buttermilk, milk, vanilla and oil and mix until just combined.
• In another large bowl whip the egg whites until you reach medium stiff peaks.
• Fold the egg whites into the batter until no streaks of whites remain. Separate the batter equally into 6 different bowls. I had about 275 grams of batter per bowl. Color the batter red, orange, yellow, green, blue, and purple.
• Bake in prepared pans for 15-18 minutes, until the center of the cake springs back when lightly pressed on.
• Let the cakes cool completely before wrapping with plastic wrap and stored flat in the freezer overnight. This will make them easier to stack and frost.

Buttercream

• Beat the butter and powdered sugar together in a stand mixer until combined. Add the vanilla, salt and 4-5 tablespoons of cream. Beat until smooth. Add more cream if needed to reach a softer, smoother consistency.
• Reserve 2 cups of white frosting, and tint the rest of it light blue.
• Start with the purple cake round on the bottom and add a thin layer of frosting. Top with the blue round followed by a thin layer of frosting. Repeat with the rest of the rounds until you have them stacked purple, blue, green, yellow, orange, and finishing with red on top. Add a thin layer of frosting over the outside of the cake to form the crumb coat. Chill for 20 minutes.
• Once the crumb coat has chilled, use a frosting spatula to smooth the frosting around the cake. Avoid lifting the spatula straight off the cake and instead swipe it along the cake until it comes off. This will help avoid pulling off the sides of the cake and getting crumbs in your buttercream.
• Use a piping bag fitted with a very large round tip to pipe white clouds around the top and cascading over the top edge of the cake.