Put the flour, hazelnuts, sugar, vanilla sugar, butter and egg yolk in a bowl and knead into a shortcrust pastry. Cover and chill for about 1 hour. Form balls (approx. 2 cm Ø) and place on 3 baking trays lined with baking paper. Press a small depression into each cookie with a floured spoon handle.
Warm the jelly briefly and stir until smooth. Using a small spoon, fill a little jelly into each cookie. Bake one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 15 minutes. Let them cool down on a grid and dust with icing sugar