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 a lightly floured work surface. Place on 2 baking trays lined with baking paper and bake in the preheated oven (electric range: 200 °C/ convection oven: 175 °C/ gas: level 3) one after the other for approx. 10 minutes, so that the tips in particular become brown.
Sift icing sugar and mix with remaining vanillin sugar. Carefully roll hot croissants in it. Leave to cool on a cake rack. Keep the croissants well closed in tins in a cool place. Keep for approx. 4 weeks.
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