Mix butter, vanillin sugar and icing sugar with the whisk of the hand mixer until creamy. Stir in eggs. Mix flour and cocoa and fold in. Pour the dough into a piping bag with a star-shaped spout and squirt 12 biscuits each onto 2 baking trays lined with baking paper.
Put the trays in a cool place for about 1 hour.
Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 8-10 minutes. Remove the cookies and let them cool down on a cake rack.