Preheat your oven to 350°F (175°C) and spread the hazelnuts on a baking sheet. Roast for about 10 minutes, then cool and chop.

In a large mixing bowl, beat together the softened butter, granulated sugar, and brown sugar until fluffy. Add the egg and vanilla extract, mix well.

Sift flour, cocoa powder, baking powder, and salt, then fold these into the wet mixture along with the chopped hazelnuts for the dough.

Scoop tablespoon-sized portions of dough and roll into discs, placing them spaced on a lined baking sheet.

Bake for 10 to 12 minutes until edges are set. Cool on the sheet for a few minutes before transferring to a wire rack.

Melt dark chocolate in a microwave-safe bowl, and dip each cooled cookie halfway into the chocolate.

Sprinkle reserved chopped hazelnuts on top of each cookie while the chocolate is soft. Let set at room temperature or refrigerate.