Put flour, almonds, hazelnuts, sugar, 1 sachet of vanilla sugar, salt and egg yolk in a bowl. Add butter in pieces. Knead with the dough hooks of the hand mixer to a smooth dough. Shape into a ball with your hands, wrap in foil and chill for about 1 hour. Roll and shape small pieces of dough into croissants on the work surface.
Place on a baking tray lined with baking paper. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 10 minutes, so that just the tips turn brown. Sift icing sugar and mix with remaining vanillin sugar. Carefully roll hot croissants in it. Let cool on a cake rack