For the nut croissants, first prepare the puff pastry.roll out the puff pastry into a rectangle of approx. 30 x 45 cm, cut in half lengthwise and cut into eight triangles each. Cut each triangle in the middle of the short side about 3 cm and let the dough relax for about 5 minutes.
For the filling, mix together chopped and ground hazelnuts, honey, cinnamon, egg whites and whipped cream. Spoon a heaping teaspoon of each onto the short side of the dough. Roll up the dough triangles from the short side, pulling the dough slightly outward with your fingers. Place on two baking sheets with baking paper and let them proof, covered, for 20 minutes.
Preheat the oven to 200°C (convection oven 180°C). Whisk egg yolk and milk and brush the croissants with it. Bake in the oven (center) for about 20 minutes until golden brown. Remove and let the nut croissants cool on a cooling rack.