From Bien Cuit bakery's pastry chef and owner Zachary Golper: “We decided to use buckwheat for this cookie because of its unique, beautiful, flavor; tender and crumbly texture; and the fact that it makes gluten-free people happy." —Food52
Preheat oven to 375° F. Line a rimmed baking sheet with parchment paper.
Whisk flour, cocoa powder, and baking powder in a medium bowl; set aside. Using an electric mixer on medium-high speed, beat brown sugar and butter in a large bowl until light and fluffy, about 4 minutes. Gradually add dry ingredients and beat on medium-low until no dry spots remain. Add chocolate chips and espresso and beat on low speed until just blended.
Transfer dough to a lightly floured work surface. Shape into a log about 2" in diameter and 10" long. If possible, chill the dough for at least 1 hour before cutting for optimal results.
After chilling, remove the dough. Cut crosswise into 12 even pieces (they will be large). Place cookies on prepared baking sheet and sprinkle with powdered sugar and a little bit of salt. Bake, rotating once halfway through, until cookies are cracked, slightly puffed, and baked through (they will still be slightly soft and firm up as they cool), 12 to 15 minutes. Let cool slightly on baking sheet, then transfer to a wire rack; cool completely.