After placing racks in the bottom and top third of your oven, preheat to 350°F. Parchment 2 baking sheets.Bake the hazelnuts on one of the prepared baking sheets for 10 minutes, until browned and your house smells nutty .
Transfer toasted hazelnuts to a clean, damp tea towel when cool enough to handle. To remove as much skin as possible, wrap them in the towel and forcefully massage for 1–2 minutes.
Hazelnut skins may be removed in a food processor. Add ¾ cup powdered sugar and 2 tablespoons to hazelnuts. Grind hazelnuts and 2 tablespoons sugar for 1–2 minutes.
Add remaining flour, salt, and sugar. Process 1 minute to evenly distribute dry ingredients. Stop and scrape the processing basin bottom using rubber spatula.Add butter cubes, vanilla, and lemon zest to dry ingredients.
Pulse a few times to break up and distribute the butter, then process until the dough coalesces, 30 seconds.Place dough on a clean surface. Make multiple gentle kneads if there are dry flour bits.
Make a 12-inch-long, 1-inch-diameter log from each piece. If the log breaks, gently push it back together. To prevent dough from sticking to the counter, gently flour it.Cut each log into 24 ½-inch pieces using a bench scraper or sharp knife.
Bake for 8 minutes, then move the pans back to front and swap oven racks. Bake the cookies for 6 more minutes until the tops are dry and the edges are softly brown.Cool the cookies in the pan for 30 minutes at room temperature.
Put chopped chocolate in a microwave-safe bowl. In the microwave, stir chocolate every 30 seconds with a rubber spatula until smooth.Add ½ teaspoon of chocolate to one cookie and gently place another biscuit on top to create a sandwich.
Get chocolate to the cookie edges. Pair the remaining cookie halves by shape and size.Let chocolate harden for 15 minutes before serving. Cookies in an airtight container may be kept at room temperature for a week.