Ingredients
Cake
- 3 ⅓ cups cake flour
- 2 ¼ cups granulated sugar
- 2 ¼ teaspoons baking powder
- ¾ teaspoon baking soda
- 1 ½ cups buttermilk
- ⅓ cup milk
- 3 teaspoons vanilla extract
- ¾ cup vegetable oil
- 6 egg whites (room temperature)
- Food coloring (red, orange, yellow, green, blue, purple)
Buttercream Frosting
- 2 pounds unsalted butter (room temperature)
- 16 cups powdered sugar
- 2 tablespoons vanilla extract
- 1 pinch salt
- ⅓ cup heavy cream (more if needed)
- Food coloring (light blue and white for decoration)
Instructions
- Prepare pans and oven: Preheat the oven to 350°F (175°C). Spray six 8-inch cake pans with cooking spray and line with parchment rounds. If you don’t have six pans, bake in batches, two cakes at a time.
- Mix dry ingredients: In a large bowl, whisk together the cake flour, granulated sugar, baking powder, and baking soda until thoroughly combined.
- Add wet ingredients: Into the dry mixture, add the buttermilk, milk, vanilla extract, and vegetable oil. Mix gently until just combined, being careful not to overmix.
- Whip egg whites: In a separate large bowl, whip the room temperature egg whites until medium stiff peaks form, giving the cake a light, airy texture.
- Fold in egg whites: Carefully fold the whipped egg whites into the batter until no streaks remain, ensuring the batter remains airy.
- Divide and color batter: Divide the batter equally into six separate bowls (approximately 275 grams per bowl). Tint each portion with a different food coloring: red, orange, yellow, green, blue, and purple.
- Bake: Pour each colored batter into the prepared pans and bake for 15-18 minutes, or until the center springs back when lightly pressed.
- Cool and freeze: Let the cakes cool completely. Wrap each layer individually with plastic wrap and store flat in the freezer overnight. This firms them up, making stacking and frosting easier.
- Prepare buttercream: In a stand mixer, beat the unsalted butter and powdered sugar until combined. Add vanilla extract, salt, and 4-5 tablespoons of heavy cream, beating until smooth. Add more cream if needed for a softer consistency.
- Tint frosting: Reserve 2 cups of white frosting for decoration. Tint the remaining frosting light blue for layering the cake.
- Assemble cake: Start stacking the cake layers from bottom to top in this order: purple, blue, green, yellow, orange, and red, spreading a thin layer of light blue frosting between each layer.
- Crumb coat: Apply a thin layer of frosting over the outside of the cake to seal in crumbs. Chill the cake for 20 minutes to set the crumb coat.
- Smooth outer frosting: Using a frosting spatula, smooth the frosting evenly around the cake by swiping along the sides without lifting the spatula straight up to avoid pulling off frosting or crumbs.
- Decorate: Using a piping bag fitted with a large round tip, pipe white cloud decorations around the top edge and cascading slightly down the sides to complete the rainbow theme.
Notes
- If you have fewer cake pans, bake the colored layers in batches, letting the pans cool completely between uses.
- Freezing the cake layers overnight makes them easier to stack and frost without crumbling.
- Room temperature egg whites whip better and create a lighter batter texture.
- Use gel-based food coloring for more vibrant colors without thinning the batter.
- Store the finished cake in the refrigerator and allow it to come to room temperature before serving for best flavor.
- Prep Time: 45 minutes
- Cook Time: 30 minutes (baking layers in batches)
- Category: Dessert
- Method: Baking
- Cuisine: American
