Preheat the oven to 350°F (175°C). Grease and line two 8-inch round cake pans with parchment paper.
In a medium bowl, whisk together the flour, baking powder, and salt.
In a large bowl, beat the softened butter and sugar until light and fluffy, about 3–4 minutes.
Add the egg whites one at a time, beating well after each addition. Mix in the vanilla extract and cotton candy flavoring.
Alternately add the dry ingredients and milk to the butter mixture, beginning and ending with the dry ingredients. Mix just until combined.
Divide the batter evenly into two bowls. Tint one bowl pink and the other blue using gel food coloring.
Spoon alternating dollops of each colored batter into the prepared cake pans. Gently swirl with a knife to create a pastel marble effect without overmixing.
Bake for 28–32 minutes, or until a toothpick inserted into the center comes out clean.
Cool the cakes in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
Beat the butter until creamy. Gradually add the powdered sugar, then mix in the heavy cream and cotton candy flavoring until smooth and fluffy.
Divide the frosting into two bowls and tint one pink and the other blue. Frost and stack the cooled cake layers, creating a soft pastel swirl effect.
Decorate with sprinkles, edible glitter, and fresh cotton candy just before serving for the best presentation.