"This is my family's favorite Christmas cookie. My mom used to make them at the beginning of Christmas season in Germany and she had to hide some so they would last until Christmas. I have tried the recipe now with American ingredients and finally got it to work."
Preheat the oven to 350 degrees F (175 degrees C). Spread the hazelnuts out in a single layer on a baking sheet. Roast in the preheated oven until toasted and fragrant, about 10 minutes. Transfer to a damp towel, cover the nuts and rub lightly to remove most of the skins. Allow nuts to cool completely.
When the hazelnuts are cool, transfer to a food processor or blender. Cover and pulse until ground, although not too fine since this will give a more interesting texture.
In a large bowl, stir together the ground hazelnuts, flour, baking powder, salt, potato starch and confectioners' sugar. Measure in the vanilla, and butter; knead everything by hand until it becomes smooth and evenly mixed dough. Divide into 8 portions and roll each one into a log about 1 inch in diameter. Stir together the cocoa powder and white sugar; place on a plate. Roll each piece of dough in the cocoa and sugar mixture. Wrap with plastic or waxed paper and place in the freezer until firm, about 45 minutes.
Preheat the oven to 375 degrees F (190 degrees C). Slice the rolls of cookie dough into 1/4 inch thick slices and place the cookies 1 inch apart on baking sheets.
Bake in the preheated oven until the centers appear dry and the bottoms are lightly toasted, about 10 minutes.