Toast the hazelnuts at 160 degrees C (320 degrees F) for 15 minutes, until golden brown and fragrant.
Remove the hazelnuts from the oven and rub them in a kitchen towel until the loose skins come off (don't worry about skins that don't come off).
Mix all the ingredients together until the butter is evenly dispersed, and the dough is smooth.
Cut off equal-sized pieces (more or less weighing 8 grams each) and roll them into tiny balls between the palms of your hands. Work with one log of dough at a time, leaving the others in the fridge, so they stay cold.
In a clean, dry bowl set over a pan of barely simmering water, melt the chocolate until smooth. Let it cool for a few minutes until it thickens slightly.
Put a dollop of chocolate on one cookie and place another one on top.
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