Mix butter, sugar and egg yolk.
Add flour, hazelnuts and 2 tsp.
cinnamon and knead everything into a dough.
Refrigerate the dough for approx. 1-2 hours. Then form croissants from the dough and place on a baking tray lined with baking paper. Bake in a preheated oven at 175°C for about 15 minutes. Roll in a mixture of cinnamon and sugar while still warm.
