Dough: Place the flour, sugar, cubed butter, and hazelnuts in the food processor. Process shortly to form a crumbly dough. Transfer the dough to the working surface and knead briefly to bring it into shape. Form a disc, wrap it tightly in plastic foil.
Refrigerate for at least one hour.
Preheat the oven to 175 degrees Celsius/ 350 degrees Fahrenheit. Line one or two baking trays with baking paper; you will have to work in batches.
Shape: Form a long roll and cut it into small pieces. To check, form one ball, it should be roughly the size of a larger cherry. Next, form the dough pieces into crescents and place them on the baking tray. Don't make the ends too thin; you don't want them to get too brown in the oven.
Bake in batches: When a tray is full, bake the first batch, in the middle of the oven, for about 13-15 minutes or until very lightly browned around the edges. While the first crescents are baking, form the next batch.
Rest: Take the crescents out of the oven and rest on the tray for 3 or 4 minutes, but not longer. Mix the sugar and the vanilla sugar on a large plate.
Coat with sugar: Carefully roll the crescents into the sugar, then place them on a wire rack to cool completely. You will probably break the first 2 or 3 until you get the hang of it. Use a kitchen scale to measure the ingredients; cup measuring is too imprecise. Ground almonds can be used instead.
Repeat the process with the remaining dough. They keep airtight in cookie jars or cookie tins for 2 or 3 weeks.