Preheat the oven to 350 (175 C) degrees and place racks in the center and upper third of the oven.
Spread the hazelnuts on a small rimmed baking sheet and toast 8-10 minutes. Cool.
Put the nuts in the bowl of a food processor and process a few seconds to roughly chop. Add the sugar and process again until finely chopped. Add the remaining ingredients and pulse just until a pebbly dough forms (you may need to scrape the bowl to incorporate all the flour).
Transfer the dough to a bowl, and knead it gently to form a ball. You can form the cookies right away, or cover and refrigerate 30 minutes.
Increase the oven temperature to 375 (190 C) degrees. Line a baking sheet with a piece of parchment paper.
Roll tablespoons of dough into balls and arrange on the baking sheets. Bake 16-18 minutes, rotating the pans from one rack to the other halfway through. The cookies should be firm and lightly golden brown, with slight cracks on their tops. Let them cool 5 minutes on the baking sheet.
One by one, roll the warm cookies in the sugar, then return to the baking sheet to set. Let stand 5 minutes before giving the cookies a second coating.
When the cookies are cool, gently toss them with the cocoa to coat.