For the hazelnut doughnuts, preheat the oven to 170º C hot air.
Mix sugar, vanilla sugar, salt and butter until very fluffy. Then add the egg yolks. Add flour and quickly mix to a dough. Leave to rest refrigerated for two hours.
Form small doughnuts from the dough and place them on a baking tray lined with baking paper. Make a small indentation in the center of the doughnuts with a wooden spoon handle and fill it with a hazelnut.
Brush the hazelnut doughnuts with a beaten egg and bake for about 15 minutes.
Preparation Tip:
A wonderful hazelnut doughnut recipe!