Melt the butter and then stir together with the eggs, 200g sour cream and sugar in a bowl. Mix in the flour and baking powder. Carefully fold in two thirds of the washed blueberries. Pour the batter into the muffin cases and sprinkle the remaining berries on top. Bake the muffins in the oven for about 10 minutes.
In the meantime, mix the remaining sour cream and vanilla sugar together. Distribute the topping evenly on the baked muffins and bake them for another 15-20 minutes.