Ingredients:

  • 3 cups (375g) cake flour, sifted
  • 1 ½ cups (300g) granulated sugar
  • 1 tbsp (12g) baking powder
  • ½ tsp (3g) salt
  • 1 cup (225g) unsalted butter, softened
  • 5 large egg whites, room temperature
  • 1 cup (240ml) whole milk, room temperature
  • 1 tbsp (15ml) clear vanilla extract
  • 2 tbsp (30g) vegetable oil
  • 2 cups (450g) unsalted butter, softened
  • 6 cups (720g) powdered sugar, sifted
  • 3 tbsp (45ml) heavy cream
  • 2 tsp (10ml) vanilla extract
  • ¼ tsp (1.5g) salt
  • Gel food coloring (Red, Orange, Yellow, Green, Blue, Purple)

Instructions:

  1. Preheat oven to 350°F (175°C). Grease six cake pans and line the bottoms with parchment paper.
  2. Beat softened butter and sugar together on medium-high speed until the mixture is pale and fluffy (about 3 minutes).
  3. Slowly add egg whites one at a time, beating well after each addition. Stir in the clear vanilla and vegetable oil.
  4. Alternately add the sifted flour, baking powder, and salt mixture and the milk, starting and ending with the flour. Mix on low until just combined.
  5. Divide the batter evenly into six bowls and stir in the red, orange, yellow, green, blue, and purple gel food colorings.
  6. Pour each colored batter into separate pans and bake for 30 minutes. Allow cakes to cool completely.
  7. Prepare the buttercream by beating together the softened butter, powdered sugar, heavy cream, vanilla extract, and salt until silky.
  8. Stack the cake layers with buttercream in between. Apply a thin crumb coat of frosting over the entire cake.
  9. Chill the cake in the refrigerator for 2 hours to set the crumb coat.
  10. Apply the final layer of buttercream frosting and decorate as desired.