On a clean work surface, combine the flour, ground hazelnuts or almonds and sugar, mix well and shape into a mound with a well in the center.
Put the egg yolks in the center of the well and the butter, cut into small cubes, around it.
Using your fingertips, work the dough from the outside in, pinching it, until it comes together into a firm dough.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Preheat the oven to 350°F (180°C).
Take the dough out of the refrigerator and divide it into pieces about the size of a walnut.
With your hands, roll each piece into a small log that is thinner at the ends. Place them on a baking sheet lined with parchment paper and curve them into the classic crescent shape.
Bake for 15 minutes.
While they bake, mix the powdered sugar with the vanilla sugar in a shallow bowl.
Set aside.
When the cookies come out of the oven, carefully place them top side down, 2 or 3 at a time, in the bowl of sugar for a few seconds so they get coated.
Let them cool, then coat the cookies in the sugar a second time, and enjoy! ¡A disfrutar!