Recipe Index

Ingredients

• 3⅓ cups cake flour
• 2¼ cups granulated sugar
• 2¼ tsp. baking powder
• ¾ tsp. baking soda
• 1½ cups buttermilk
• ⅓ cup milk
• 3 tsp. vanilla extract
• ¾ cup vegetable oil
• 6 egg whites, room temperature
• Food coloring
• 2 lb. unsalted butter, room temperature
• 16 cups powdered sugar
• 2 Tbsp. vanilla extract
• 1 pinch salt
• ⅓ cup heavy cream, more if needed
• Food coloring

Method

1. Heat the oven to 350°F, spray six 8-inch cake pans with cooking spray and line them with parchment rounds, baking in batches if fewer pans are available.
2. Whisk the cake flour, granulated sugar, baking powder and baking soda in a large bowl, then mix in the buttermilk, milk, vanilla and vegetable oil until just combined.
3. Whip the egg whites in a clean large bowl to medium-stiff peaks.
4. Fold the whipped whites through the batter until no white streaks remain.
5. Divide the batter equally among six bowls and tint them red, orange, yellow, green, blue and purple.
6. Bake each layer for 15 to 18 minutes, until the center springs back when pressed lightly.
7. Cool the layers completely, wrap them flat, and freeze them overnight so they stack and frost cleanly the next day.
8. Beat the butter and powdered sugar together until combined, add the vanilla, pinch of salt and 4 to 5 tablespoons of the cream, and beat until smooth, adding a little more cream if the buttercream needs softening.
9. Set aside 2 cups of the white buttercream and tint the rest pale blue.
10. Build the cake starting with the purple layer, spreading a thin coat of frosting between each layer, and continue in order — blue, green, yellow, orange — finishing with red on top; cover the whole cake with a thin crumb coat and chill it until firm.
11. Smooth the blue buttercream over the chilled cake, drawing the spatula along the surface rather than lifting it away, so crumbs stay out of the finish.
12. Pipe the reserved white buttercream from a bag fitted with a very large round tip in clouds around the top, letting them cascade slightly over the edge.

Prep: 60 min · Cook: 18 min · Total: 318 min

Nutrition (per serving, where published)

Estimated from USDA FoodData Central ingredient data