In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt.
In the bowl of a standup mixer fitted with the paddle attachment, cream together the butter and granulated sugar on medium speed until pale yellow and airy, about 5 minutes. Add the egg and vanilla and beat just until incorporated. Reduce the mixer speed to low, add the flour mixture and mix until evenly combined.
Divide into two equal pieces. Shape each mound of dough into a ball, flatten into a disk, wrap in plastic wrap, and chill for at least 1 hour or overnight.
Preheat the oven to 350° and line a rimmed baking sheet with parchment paper. On a floured surface, use a rolling pin to roll one of the chilled cookie dough discs into a ¼-inch thick sheet. Dough will be slightly dry but gently roll it out. Use a 2½-inch round cookie cutter (or any other shape) to cut the dough and place them 2 inches apart on the prepared baking sheet.
Bake the cookies until their edges are firm and the centers are slightly soft and puffed, 8 to 10 minutes. Use a metal spatula to transfer the cookies to a wire rack to cool completely. Repeat with the remaining cookie dough. Cool the cookies completely before filling.
Spread preserves in the center of half of the cookies. Do not spread it too close to the edge. Top each filling-topped half with another cookie and press down slightly to evenly distribute the filling. Sprinkle with confectioners’ sugar. Serve immediately.
Yield: 16 cookie sandwiches
Cook Time: 10 minutes (plus 1 hour to chill dough)