01 -
Preheat oven to 375°F. Line two baking sheets with parchment paper.
02 -
In a food processor fitted with the S blade attachment or in the bowl of an electric mixer fitted with the paddle attachment, combine flour, cocoa powder, baking soda, baking powder, salt, and sugar. While pulsing, or on low speed, add the butter in several pieces at a time, then add the egg. Continue processing, or mixing, until the dough comes together in a mass, about 1 minute.
03 -
Divide the dough into 1-tablespoon sized balls using a small cookie scoop and drop onto prepared baking sheets, at least 2 inches apart. Dampen the palm of your hand before flattening each ball of dough to about 1/8-inch thickness. Chill each tray of flattened dough for at least 10 minutes in the fridge before baking.
04 -
Bake for 9 to 10 minutes, rotating once to ensure even baking, or until the cookies are fragrant and completely set. Set baking sheets on a rack to cool.
05 -
In a stand mixer fitted with the paddle attachment, beat the butter and vanilla on medium-low speed until well combined. Scrape down the bowl. Gradually add the powdered sugar, then the salt. Turn the mixer on high and beat for 2-3 minutes until filling is light and fluffy.
06 -
Using a pastry bag or spatula, drop a teaspoon-size amount of filling into the center of one cookie. Place another cookie, equal in size to the first, on top of the filling. Lightly press down to work the filling evenly towards the outside of the cookie. Repeat with remaining cookies and filling. Serve the same day for best texture.