Preparation: Preheat the oven to 350°F (175°C). Grease and flour three 6-inch round cake pans and line the bottoms with parchment paper.

Dry Ingredients: In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

Creaming: In a stand mixer fitted with the paddle attachment, cream the unsalted butter and granulated sugar on medium-high speed until pale and fluffy, approximately 3 minutes.

Add Eggs: Reduce the mixer speed to low and add the eggs one at a time, mixing well after each addition.

Flavoring: Add the cotton candy flavoring and mix until incorporated.

Combine Wet and Dry Ingredients: Alternately add the dry flour mixture and the milk to the batter, beginning and ending with the flour mixture (three additions of flour and two of milk). Mix until just combined; avoid overmixing.

Coloring the Batter: Divide the batter evenly into four bowls. Leave one portion uncolored and tint the remaining three with fuchsia, sky blue, and violet gel food coloring, respectively.

Marbling: Spoon dollops of each colored batter into the prepared pans, layering and alternating colors. Use a toothpick or skewer to gently swirl the batters together to create a marbled effect.

Baking: Bake in the preheated oven for approximately 35 minutes, or until a toothpick inserted into the center of the cakes comes out clean.

Cooling: Allow the cakes to cool in the pans for 10 minutes before turning them out onto a wire rack to cool completely.

Egg White Mixture: Place the egg whites and granulated sugar into the bowl of a stand mixer and whisk until combined.

Heating: Set the bowl over a pot of simmering water (double boiler) and whisk constantly until the mixture is hot and no longer grainy to the touch, approximately 3 minutes, or until it reaches 160°F (71°C) on a candy thermometer.

Whipping Meringue: Attach the bowl to the stand mixer fitted with the whisk attachment. Beat on medium-high speed until the meringue is stiff and has cooled to room temperature, about 5-10 minutes.

Adding Butter: Switch to the paddle attachment. With the mixer on low speed, add the room temperature butter one cube at a time, mixing well after each addition.

Flavoring: Once all the butter is incorporated and the buttercream is smooth, add the cotton candy flavoring and mix until combined.

Leveling: If necessary, level the tops of the cooled cakes to ensure they are flat.

Layering: Place one cake layer on a serving plate or cake stand. Spread approximately 2/3 cup of buttercream evenly over the top. Repeat with the second layer. Place the third layer on top.

Crumb Coat: Apply a thin layer