INGREDIENTS
6 tbsp
unsalted butter, softened
1 cup
packed dark brown sugar
1 tsp
pure vanilla extract
1
large egg, at room temperature
1
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
5
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