Mix the butter, sugar and vanilla sugar until frothy. Mix in the eggs one by one. Mix flour with baking powder and salt as well as cocoa. Sift into the butter mixture and stir in the milk.
Half fill the paper cases with batter. Place a chocolate cube on each one and use the spoon to spread the rest of the batter evenly on top.
Bake in the oven preheated to 190 ° C for 20 minutes.