Soak the whole hazelnuts in warm water at least an hour in advance.
Beat the butter, yolk, and powdered sugar into a smooth mixture.
Add all-purpose flour and vanilla essence.
Knead the dough with your hands quickly.
Take out the nuts, let them dry off, and knead them into the dough.
Split the dough into 2 halves, and make a thin loaf about 1.5-2 inches (3-5 cm) thick from both. Wrap them in a plastic foil and leave them to rest in the fridge, ideally overnight.
Preheat the oven to 170 °C (340 °F).
Slice each loaf into slices about ½ inch (1,5 cm) thick.
Lay them on a baking paper-lined tray and let them bake for 15 minutes until slightly golden.
Take the tray out of the oven. Coat the cookies from all sides with powdered sugar.