First of all, put the whole hazelnuts in a mug and pour lukewarm water over them. Let them stand for about an hour. The nuts will soften slightly in the water.
In a large mixing bowl, add the flour, sugar, vanilla paste, egg yolk, and butter, chopped into smaller pieces.
Drain the nuts and dry them gently with a kitchen towel. Add them to the bowl with the other ingredients.
Make the cookie dough. First, mix the wet ingredients, i.e. egg yolk and vanilla paste, with the flour using a fork. Then, using your fingers, work the butter into the other ingredients until lumps form.
Dump the mass onto a work surface and use your hands to finish it into a smooth, slightly sticky dough. Be patient, and don't work hastily. The dough may seem too powdery at first, but as you go on, the butter will soften, and everything will come together beautifully.
Note: Don't knead the dough too long! Once you get a smooth dough, stop working it.
Divide the dough in half and roll it into elongated cylinders nearly two inches (5 cm) in diameter. Cover them in plastic wrap and let them firm up in the fridge for about an hour.
Using a sharp knife, cut the cooled cylinders of dough into rounds about ¼ inch (0.6 cm) thick.
Line a baking sheet with parchment paper. I always grease the baking sheet a little to ensure the paper sticks well. Place the cookies on the lined baking sheet and bake in a preheated oven at 340 °F (170 °C) for 13 minutes.
Let the cookies cool for about 5 minutes. While still warm, roll them in powdered sugar, scented with the vanilla pod (optional).