Cream butter with sugar, vanilla extract and salt with the whisk of the hand 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
Alternate spoonfuls of dark and light dough in a bowl. Use a teaspoon to place small heaps on about 5 baking trays lined with baking paper. Keep your distance, as the biscuits will run apart. Bake one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 10-12 minutes. Allow to cool briefly, then place on a cake rack and allow to cool