For the chocolate muffins, preheat the oven to 190°C (170°C convection oven). Grease the muffin pan wells or line with paper baking cups.
In a medium bowl, thoroughly mix the dry ingredients, i.e. flour, baking powder, baking soda, the chocolate chips and cocoa.
In another large bowl, crack the egg and whisk lightly. Then add the sugar, vanilla sugar, oil and milk and stir briefly.
Lastly, add the dry ingredients and gently fold in. Mix only until the dry ingredients are moistened. Pour the batter into the ramekins and bake at 180 °C (convection oven 160 °C) on the middle rack for 20-25 minutes.
Let the muffins cool in the baking tray for 5 to 10 minutes, then remove them from the tray and let them cool on a cooling rack. Then sprinkle with powdered sugar.