Step 1

Preheat your oven. Arrange racks in the bottom and upper third of your oven, then preheat to 350°F. Line 2 baking sheets with parchment paper.

Step 2

Roast the hazelnuts. Spread the hazelnuts out on one of the prepared baking sheets and bake for 10 minutes, until browned and your whole kitchen smells nutty (leave your oven on, we’ll bake the cookies at the same temperature).

Step 3

Remove the hazelnuts’ skins. While the hazelnuts are still warm but cool enough to handle, transfer them to a clean, damp tea towel. Wrap them in the towel and rub them together vigorously for 1 to 2 minutes to remove as much of the skins as possible.

Step 4

Grind the hazelnuts. Transfer the hazelnuts to a food processor and discard the skins. Measure out the 3/4 cup of powdered sugar, then scoop out 2 tablespoons of the sugar to add to the hazelnuts. Process the hazelnuts and 2 tablespoons of sugar until they’re finely ground, 1 to 2 minutes.

Step 5

Mix the dry ingredients. Add the remaining sugar, flour, and salt. Process until the dry ingredients are evenly distributed, about 1 minute. You may need to pause and use a rubber spatula to scrape the bottom corner of the processor bowl.

Step 6

Make the dough. Add the cubes of butter, vanilla extract, and lemon zest to the dry ingredients. Pulse a few times to break up and distribute the butter, then process until the dough begins to clump together into a cohesive dough, about 30 seconds.

Step 7

Portion the dough. Dump the dough onto a clean work surface. Gently knead the dough a few times if there are still some dry bits of flour. Divide the dough into 4 equal pieces, about 4-ounces (115g) each.

Step 8

Shape the dough. Gently shape each piece into a 1-inch diameter log about 12-inches long. If the log starts to break apart, gently press it back into shape. The dough shouldn’t stick to the counter, but if it does dust lightly with flour.

Step 9

Shape the cookies. Use a bench scraper or a sharp knife to cut each log into 24 1/2-inch pieces. Form each piece into a ball with your hands. Space them 1 inch apart on the prepared baking sheets, reusing the pan and parchment from roasting the hazelnuts. You should be able to fit 48 cookies per pan.

Step 10

Bake. Bake for 8 minutes, then rotate the pans back to front and switch the pans between the top and bottom oven racks. Continue baking until the tops of the cookies are dry and the edges are just starting to turn golden, about 6 minutes more.

Step 11

Cool. Let the cookies cool in the pan to room temperature, about 30 minutes.

Step 12

Melt the chocolate. Add the chopped chocolate to a microwave-safe bowl. Microwave the chocolate in 30-second intervals, stirring with a rubber spatula each time, until the chocolate is melted and smooth.

Step 13

Assemble. Dollop about 1/2 teaspoon of chocolate on the bottom of a cookie, then gently press another cookie on top to form a sandwich. The chocolate should spread right to the edges of the cookies. Repeat with the rest of the cookies, pairing the cookie halves by shape and size.

Step 14

Rest and serve. Let the chocolate set until firm before serving, at least 15 minutes. The cookies will keep in an airtight container at room temperature for up to 1 week.