Inside-Out Neapolitan Cupcakes

INGREDIENTS

6 tbsp

unsalted butter, softened

1 cup

packed dark brown sugar

1 tsp

pure vanilla extract

large egg, at room temperature

egg yolk, at room temperature

3/4 cup

+ 2 teaspoons (115 g) all-purpose flour

6 tbsp

unsweetened Dutch-processed cocoa powder

1/2 tsp

baking soda

1/2 tsp

baking powder

1/2 tsp

kosher salt

3/4 cup

buttermilk, at room temperature

2 tbsp

mayonnaise

large fresh egg whites (150 g)

1 1/4 cups

superfine granulated sugar

1 1/2 cups

(3 sticks) unsalted butter, softened, cut into cubes

2 tsp

pure vanilla extract

salt