1 stick unsalted butter, softened 1½ cups granulated sugar 3 large eggs 2⅓ cups all-purpose flour ⅓ cup cocoa powder, sifted (plus extra to shape cookies) 1 teaspoon baking powder Pinch of salt
¾ cup heavy cream 8 ounces semisweet chocolate, chopped
Preheat oven to 325˚. Line four baking sheets with parchment paper, and set aside. Place butter and sugar in the bowl of a stand mixer fitted with the paddle attachment, and beat until creamy. Add eggs, and beat. Add flour, cocoa powder, baking powder, and salt; mix until combined and dough comes together.
Shape the cookies: Using cocoa powder-dusted hands, roll about 2 teaspoons of dough into a ball. Place ball on baking sheet and flatten slightly. Repeat until all dough is used, placing about 12 cookies on each baking sheet. Bake in batches until cookies are set, about 15 to 18 minutes. Let cool. While the cookies cool, make the filling. Heat heavy cream in a small saucepan until just simmering. Remove from heat; add chocolate. Stir until smooth and chocolate is melted; transfer to a small bowl. Chill in refrigerator about one hour, or until filling is thickened and resembles the consistency of store-bought icing.
To finish the cookies, divide and spread the filling equally onto the bottoms of half of the cookies. Top with the plain cookies to create sandwiches.