Preheat oven to 350°F (175°C). Grease 2–3 8-inch cake pans and line with parchment paper; lightly flour sides.

In a medium bowl, whisk together cake flour, baking powder, and salt. Set aside.

Beat butter and sugar in a large bowl with a mixer until light and fluffy, about 3–4 minutes.

Add eggs one at a time, mixing well after each addition. Stir in vanilla extract and cotton candy syrup.

Alternately add dry ingredients and milk to the butter mixture, beginning and ending with dry ingredients. Mix until just combined.

Divide batter into two bowls and add pink food coloring to one, blue to the other. Gently swirl in pans for a marbled effect.

Pour batter into prepared pans and bake 25–30 minutes, or until a toothpick comes out clean. Cool in pan 10 minutes, then transfer to a wire rack.

Beat butter for frosting until fluffy. Gradually add powdered sugar, then heavy cream, vanilla, and cotton candy syrup. Beat until light and airy.

Level cake layers, spread frosting between layers, apply a crumb coat, and chill 20 minutes before final frosting.

Smoothly frost the cake, decorate with sprinkles, glitter, or cotton candy pieces, and serve immediately or refrigerate.