For the Whoopie Cake: Arrange the oven racks in the upper and lower thirds of the oven and preheat the oven to 425°F. Line 2 large baking sheets with parchment paper.
1 cups Unbleached All-Purpose Flour 1/4 cup Unsweetened Cocoa 1/4 teaspoon Baking Soda 1/4 teaspoon Fine Salt
Whisk together the flours, cocoa, baking soda and salt in a medium bowl.
4 tablespoons Unsalted Butter (at room temperature) 1/2 cup Granulated Sugar 1 large Egg 1/2 cup Milk
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.
1 pint Mascarpone Cheese 1/2 cup Confectioners Sugar 1 teaspoon Vanilla 1/4 cup Blueberry Puree (strained) pinch fine salt
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 touch. Let cool on the sheets for 5 minutes, and transfer to racks to cool completely.
For the Filling: Beat together mascarpone, confectioners sugar, salt and vanilla in a bowl with electric mixer at medium speed until smooth, about 3 minutes. Fold in blueberry puree.
Spread filling on half of the cakes. Sandwich them closed and serve.
For Blueberry Puree: Pulse a pint of blueberries in a blender or food processor until pureed. Strain mixture with strainer or cheese cloth.