01 -
Preheat the oven to 350°F/177°C. Spray three 6-inch or two 8-inch cake pans with baking spray and line the bottoms with parchment paper circles.
02 -
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
03 -
In a stand mixer fitted with the paddle attachment, beat the butter at high speed until creamy—about 2 minutes. Add the sugar and beat at medium-high speed until light and fluffy, scraping the bowl and paddle as needed.
04 -
Reduce mixer speed to low. Add egg whites one at a time, mixing until just combined. Scrape the bowl and paddle. Add vegetable oil and cotton candy flavor, then beat on high speed for 1 minute.
05 -
Turn the mixer off. Add the dry mixture and mix on low speed until just combined. Gradually pour in the buttermilk and mix until uniform and slightly thin.
06 -
Divide batter evenly into two bowls. Add pink food coloring to one bowl and blue to the other. Stir until colors are uniform.
07 -
Alternately layer pink and blue batter into prepared pans using a 1/2 cup measuring cup. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Let cakes cool for 5 minutes in the pans, then transfer to a wire rack to cool completely.
08 -
In a mixer, cream the butter on medium-high until light and fluffy—about 7 minutes. Gradually add powdered sugar at low speed, scraping down after each addition. Add milk and salt, mixing until smooth. Reserve 1 cup of buttercream if making pastel colors.
09 -
Mix cotton candy flavor into the remaining buttercream. If making pastel colors, divide reserved buttercream into two bowls and tint with purple and blue food coloring as desired.
10 -
Level cake layers and stack them with about 3/4 cup of buttercream between each layer. Crumb coat and refrigerate for 30 minutes. Frost with pastel and pink buttercream for a watercolor finish. Add decorative swirls using a piping bag.
11 -
Top the cake with a generous amount of cotton candy just before serving, as it will begin to shrink after one hour.