Beat the sugar, vanilla sugar, eggs and butter until frothy. Add buttermilk and keep stirring. Mix in flour with baking powder and stir in until everything is connected. Then chop 100 g chocolate and fold it into the batter together with the blueberries. Fill into muffin molds and bake at 170 degrees for about 20 minutes.
During the time, stir a smooth mixture from the cream cheese and powdered sugar and color it purple with food coloring (goes best with the color of the blueberries). Then use a piping bag to drape the mixture onto the cooled muffins. Finally, make chocolate chips out of the remaining 50 g of chocolate and sprinkle over the cupcakes.
It is essential to store in the refrigerator so that the topping becomes firmer. 30-60 minutes are enough.