Cream butter, sugar, vanilla sugar and 1 pinch of salt with the whisks of the mixer. Stir in the eggs one by one. Mix flour and baking powder, stir in briefly. Halve the dough. Stir in one half cocoa and milk.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Line two baking trays with baking paper. Place alternately dark and light pastry in a bowl. Pull through with a fork to form a light marble pattern.
From approx. 1⁄3 Cut off the dough with a teaspoon about 10 small piles of dough at a time and place them on the trays. Leave plenty of space between the individual piles of dough, as the dough runs very far apart.
Bake cookies in the oven for 10-12 minutes one after the other. Put the rest of the dough aside.
Allow cookies to cool briefly on the trays, then let them cool down on a cake rack. Spread the rest of the dough on four more trays, process and bake in the same way.