Ingredients:

  • 420g cake flour
  • 400g granulated sugar for the sponge
  • 1.5 tbsp baking powder
  • 1 tsp salt
  • 225g unsalted butter, room temperature
  • 360ml whole milk
  • 6 large egg whites for the sponge
  • 2 tsp pure vanilla extract
  • 1 tsp red gel food coloring
  • 1 tsp orange gel food coloring
  • 1 tsp yellow gel food coloring
  • 1 tsp green gel food coloring
  • 1 tsp blue gel food coloring
  • 1 tsp purple gel food coloring
  • 6 large egg whites for the buttercream
  • 400g granulated sugar for the buttercream
  • 680g unsalted butter, cubed and cool
  • 1 tbsp vanilla bean paste

Instructions:

  1. Preheat oven to 350°F (180°C) and grease six cake pans thoroughly. <small>Note: Proper greasing ensures the delicate sponge doesn't tear during release.</small>
  2. Sift 420g cake flour, 400g sugar, 1.5 tbsp baking powder, and 1 tsp salt into your mixer bowl.
  3. Add 225g room temperature butter and mix on low until the mixture looks like fine damp sand.
  4. In a separate jar, whisk 360ml whole milk, 6 egg whites, and 2 tsp vanilla extract until combined.
  5. Gradually pour half of the liquid into the flour mixture, beating on medium speed for 2 minutes.
  6. Fold in the remaining liquid in two batches until the batter is smooth and velvety.
  7. Divide the batter equally into six bowls, approximately 230g of batter per bowl.
  8. Add 1 tsp of each gel color (red, orange, yellow, green, blue, purple) to the individual bowls.
  9. Fold the color in gently until no white streaks remain.
  10. Pour the colored batters into the prepared pans and smooth the tops with a spatula.
  11. Bake for 25 to 30 minutes until a toothpick comes out clean and the edges pull away.
  12. Cool in the pans for 10 minutes, then invert onto a wire rack to cool completely.
  13. Prepare the buttercream by heating 6 egg whites and 400g sugar over a double boiler to 160°F (71°C).
  14. Whisk into a stiff meringue, then slowly add 680g cubed butter until the frosting is thick and silky.
  15. Stack the layers in reverse rainbow order (purple on bottom), spreading a layer of buttercream between each.