In a separate bowl, whisk the flour, cocoa powder, baking soda, espresso powder, and salt together until combined. On low speed, slowly mix into the wet ingredients until combined. The cookie dough will be quite thick. Switch to high speed and beat in the chocolate chips. The cookie dough will be sticky. Cover dough tightly with aluminum foil or plastic wrap and chill for at least 3 hours and up to 3 days. Chilling is mandatory for this cookie dough. I always chill mine overnight.