In a bowl, beat the butter with the sugar until frothy. One after the other, add the egg, oatmeal, milk and vanilla sugar while stirring and continue to stir until a smooth dough is formed. Sieve the flour, cocoa and baking powder and stir into the batter. Fold in the chocolate pieces.
Place teaspoon-sized portions of dough on a baking sheet and flatten it with a fork. Bake in the oven for 15 minutes until the cookies have risen and set. Let cool on the baking sheet for 2 minutes, then place on a wire rack and let cool down completely.