Step 1

Adjust oven rack to middle position and heat oven to 325 degrees. Line 2 rimmed baking sheets with parchment paper. Line bottom of 8" square baking pan with parchment. Process hazelnuts, flour, sugar, and salt in food processor until hazelnuts are very finely ground, 20-25 seconds. Add butter and pulse until dough just comes together, 20-25 pulses.

Step 2

Transfer dough to counter, knead briefly to form smooth ball, place in prepared pan, and press into even layer that covers bottom of pan. Freeze for 10 minutes. Run knife or bench scraper between dough and edge of pan to loosen. Turn out dough onto counter and discard parchment.

Step 3

Cut dough into 64 squares (8 rows by 8 rows). Roll dough squares into balls and evenly space 32 dough balls on each prepared sheet. Bake, 1 sheet at a time, until cookies look dry and are fragrant (cookies will settle but not spread), about 20 minutes, rotating sheet halfway through baking. Transfer sheet to wire rack and let cookies cool completely, about 30 minutes.

Step 4

Microwave chocolate in small bowl at 50 percent power, stirring every 20 seconds, until melted, 1-2 minutes. Let chocolate cool at room temperature until it is slightly thickened and registers 80 degrees, about 10 minutes. Invert half of cookies on each sheet. Using ¼ teaspoon measure, spoon chocolate onto flat surfaces of all inverted cookies. Top with remaining cookies, pressing lightly to adhere. Let chocolate set for at least 15 minutes before serving. (Cookies can be stored in airtight container at room temperature for up to 10 days.)