Preheat the oven to 180 degrees C. Spread the hazelnuts out in a single layer on a baking tray. 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 flour and icing sugar. Measure in the vanilla and butter; knead everything by hand until it becomes a smooth and evenly mixed dough. Divide into 8 portions and roll each one into a log about 3cm 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 190 degrees C. Slice the rolls of dough into 5mm thick slices and place the biscuits 3cm apart on baking trays.
Bake in the preheated oven until the centres appear dry and the bottoms are lightly toasted, about 10 minutes.