01 -
Preheat oven to 375°F. Line two baking sheets with parchment paper. In a food processor or stand mixer, combine flour, cocoa powder, baking soda, baking powder, salt, and sugar. Slowly add butter in pieces, then add the egg. Process or mix until a cohesive dough forms, about 1 minute.
02 -
Divide dough into 1-Tablespoon portions using a small cookie scoop. Roll into balls and place on baking sheets at least 2 inches apart. Flatten the dough balls to about 1/8-inch thickness using damp palms. Chill trays in the fridge for at least 10 minutes before baking.
03 -
Bake for 9 to 10 minutes, rotating the baking sheets midway. The cookies are done when fragrant and fully set. Cool completely on a rack. Store in an airtight container for up to 2 days, though cookies may soften over time.
04 -
In a stand mixer, beat butter and vanilla on medium-low speed until smooth. Gradually add powdered sugar and salt. Increase speed to high and beat for 2-3 minutes until the mixture becomes light and fluffy.
05 -
Place a teaspoon-sized amount of filling onto the center of one cookie. Top with another equal-sized cookie and gently press to spread the filling to the edges. Repeat for all cookies. Serve the same day for the best texture.