(Yields aprox 20-24 sandwich cookies)
For the cookies:
1¼ cups all-purpose flour
½ cup unsweetened Dutch process cocoa powder
1 tsp. baking soda
¼ tsp. baking powder
¼ tsp. salt
1-1½ cups sugar*
10 tbsp. (1¼ sticks) unsalted butter, at room temperature
1 large egg
For the filling:
4 tbsp. (¼ cup) unsalted butter, at room temperature
¼ cup vegetable shortening
2 cups confectioners' sugar, sifted
2 tsp. vanilla extract
Preheat the oven to 375˚ F. Line two baking sheets with silicone baking mats or parchment paper.
In a food processor or the bowl of an electric mixer, combine the flour, cocoa powder, baking soda, baking powder, salt, and sugar. Mix briefly to combine. Add the butter to the bowl and pulse or mix briefly to incorporate. Add in the egg and mix until the dough forms a cohesive mass.
Scoop a scant tablespoon of batter onto the baking sheet. Continue scooping, spacing the dough balls a couple of inches apart. With moistened fingers, gently press down on the dough balls to flatten slightly. Bake for 9-11 minutes, rotating the pans halfway through baking. Transfer the baking sheets to a wire rack and let cool completely until the cookies are firm and set. Be sure the cookies are completely cooled before filling.
To make the filling, combine the butter and shortening in the bowl of an electric mixer. Beat on medium-high speed until smooth. Add the confectioners' sugar to the bowl and mix on low speed just until incorporated, then increase the speed to medium-high and beat until light and fluffy, about 2-3 minutes. Blend in the vanilla extract.
To assemble the cookies, pair them up by size. Use a pastry bag fitted with a plain round tip to pipe a swirl of filling onto the flat side of one cookie of each pair. Sandwich together with the remaining cookie and press down so that the filling reaches the edges.