Toast and skin the hazelnuts. Preheat the oven to 350°F (175°C). Spread hazelnuts on a baking sheet and toast 8–10 minutes until fragrant. Rub warm nuts in a clean kitchen towel to remove most skins. Cool completely.

Grind the nuts. Pulse the cooled hazelnuts with the sugar in a food processor until very fine, like sand. Avoid overprocessing into paste. A few tiny flecks are fine, but the texture should be powdery.

Make the dough. Add flour and salt to the processor and pulse to combine. Scatter in the cold butter and vanilla. Pulse just until the mixture clumps and holds when pressed. The dough should be soft but not sticky.

Chill the dough. Turn the dough onto a sheet of plastic wrap. Press into a 3/4-inch-thick slab, wrap, and refrigerate 1 hour. Chilling prevents spreading and keeps the cookies round.

Portion the dough. Line two baking sheets with parchment. Using a teaspoon measure, scoop small portions (about 6–7 g each, roughly 1 teaspoon). Roll into smooth balls with cool hands. Size matters: smaller balls bake evenly and pair neatly.

Chill again. Place the dough balls on the sheets, spaced 1 inch apart. Refrigerate 20–30 minutes while the oven preheats to 325°F (165°C).

Bake. Bake one sheet at a time for 12–15 minutes, until the bottoms are lightly golden and the tops feel set. They should not deeply brown. Cool on the sheet 5 minutes, then transfer to a rack to cool fully. Repeat with the second sheet.

Prepare the chocolate filling. Melt the chopped dark chocolate in a heatproof bowl set over a pan of barely simmering water, stirring until smooth. Or microwave in short bursts, stirring often. Let it cool slightly until thick but still spreadable.

Assemble. Pair cookies of similar size. Place a small dollop of chocolate (about 1/2 teaspoon) on the flat side of one cookie and gently press its partner on top to form a sandwich. Let the chocolate set at room temperature until firm, 30–60 minutes.

Serve. Enjoy with coffee or tea. Their texture is best after the chocolate has fully set, when the cookies are crisp and the center is pleasantly snappy.