Cut the butter into cubes 1 inch large. In a medium bowl of standing mixer, mix together flour, sugar and finely ground hazelnuts. Add in the butter and beat with mixer on low speed until dough comes together in a ball.
Form the dough into a cylinder and wrap in plastic foil. Let cool in the refrigerator for at least 1 hour.
Preheat oven to 177C / 350 F. Rack in the middle position.
Remove the dough from the refrigerator and cut it into three equal pieces. Sprinkle little of a flour over the working surface and start kneading gently the first piece of the dough until it's lightly soft.
Roll the first piece into a small cylinder, approximately 1 inch in diameter and cut it into 3/4 inch slices. Take each slice and roll gently in between the palms of your hands and form a rope, about 3 inches long. Curve it gently to create the shape of crescent.
Place the crescent onto a parchment-lined baking sheet. Bake for 10-12 minutes.
In a medium bowl, mix icing sugar & vanilla sugar. Roll the crescents in the mixture of sugars while hot as soon as they are not too hot to handle.