Preheat the oven to 180 ° C. Mix the flour with the cocoa powder, baking powder and baking soda. Whisk the egg and mix with the sugar, vanilla sugar, oil, sour cream and coffee.
Grease the muffin tray. Add the flour mixture to the egg mixture and stir until the dry ingredients are moist. Fill 1/3 of the recesses in the muffin tray with the batter. Break the chocolate into pieces and place on the dough. Spread the rest of the dough on top, smooth it out and bake everything on the middle shelf for about 20-25 minutes.
Leave the muffins in the tin for another 5 minutes, remove them, allow to cool and serve sprinkled with powdered sugar.