Preheat the oven to 350°F (175°C). Grease and line three 8-inch round cake pans with parchment paper for easy release.

In a medium bowl, whisk together the flour, baking powder, and salt.

In a large mixing bowl, cream the butter and sugar together on medium-high speed for 3-4 minutes until pale and fluffy.

Incorporate the egg whites one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the flour mixture in three parts, alternating with the milk. Start and end with the flour mixture, mixing just until combined.

Divide the batter evenly into three bowls. Leave one uncolored, tint one with pink food coloring and the last with blue. Pour each into a separate prepared pan.

Bake for 25-28 minutes, or until a toothpick inserted into the center comes out clean. Cool in pans for about 10 minutes before carefully inverting onto racks.

In a clean bowl, beat the reserved butter until smooth. Gradually add the powdered sugar, mixing, followed by the milk, cotton candy flavoring, and a pinch of salt.

Tint portions of the buttercream pink and blue for decoration.

Set aside 1/2 cup of the plain buttercream and add more cotton candy flavoring if desired.

Start with the uncolored layer, spreading a layer of marbled filling and repeating with the pink and blue layers.

Spread a thin layer of buttercream over the entire cake and chill for 20 minutes to set.

After chilling, apply the final layer of frosting and decorate with piped swirls. Optionally, garnish with edible glitter or cotton candy for a whimsical look.