- 1 1/4 cups unbleached all-purpose flour
- 3/4 cup whole wheat flour
- 1/2 cup unsweetened cocoa
- 1/2 tsp. baking soda
- 1/2 tsp. fine salt
- 1 stick room temperature unsalted butter
- 1 cup granulated sugar
- 1 large egg
- 1 cup milk
- 1 stick softened unsalted butter
- 1 2/3 cups confectioners' sugar
- 1/2 tsp. vanilla extract
- 2 cups marshmallow cream
To make cakes: Whisk together the flours, cocoa, baking soda and salt in a medium bowl.
Cream the butter and sugar in the bowl of an electric mixer at high speed until fluffy, about 4 minutes. Add the egg and milk and beat at medium speed until incorporated. At low speed, add the flour mixture in 2 batches, alternating with the milk, mixing until just blended.
Using a half-tablespoon measure, drop 18 generous teaspoons of batter onto each sheet, leaving about 2 inches between cakes. Bake the 2 sheets at the same time, 5 to 7 minutes, until springy to the touch. Let cool on the sheets for 5 minutes, and transfer to racks to cool completely. Change the parchment and repeat using the remaining batter (72 cakes in total).
To make filling: Beat the butter and confectioners' sugar at low speed until blended and then beat at high speed until fluffy, about 5 minutes. Add the vanilla and marshmallow cream and mix at low speed until blended, about 1 minute.
Match the pairs of cakes with the same shapes and spread the bottom side of 1 cake with filling and sandwich together with the other cake. (Store the finished whoopee pies in a covered plastic container and chill for up to 3 days.)