Preheat oven to 325 °F (160 °C). Line baking sheets with parchment paper.

Grind hazelnuts in a food processor until very fine, resembling coarse polenta. Add sugar, salt, and flour; pulse to combine

Add cold butter and pulse until dough just begins to hold together, about 20–25 pulses

Transfer to work surface and knead briefly into a smooth ball. Wrap and chill for about 30 minutes.

Roll dough into small 1 cm (marble‑sized) balls. Place on baking sheets about 1.5″ apart. Chill again (30–60 min) for firm shape

Bake cookies for 10–15 minutes until just golden on top; for a delicate pale finish, bake at 150 °C (130 °C fan)/300 °F for 10 min then reduce to 120 °C (110 °C fan)/250 °F for an additional 5 min

Let cool completely. Melt chocolate over a bain‑marie or microwave gently. Let cool slightly until thickened (~10 min).

Invert half the cookies (flat side up), pipe or spoon chocolate onto each, then top with a second cookie to form a sandwich. Press gently and let set until firm (~15–30 min)