Whisk wet ingredients:
Combine the egg, granulated sugar, vanilla sugar, oil, and milk in a bowl, whisking until the mixture is smooth.Fold in dry ingredients:
Sift the flour, baking powder, and salt over the egg mixture, then fold gently to form a smooth batter.Divide and add cocoa:
Split the batter equally into two bowls. To one portion, add the cocoa powder and 2 tbsp milk, stirring until fully incorporated.Fill liners with batters:
Half-fill muffin liners with the light batter. Set one Kinder Schokobon in the center of each liner, then cover with the dark batter, filling the liners to about 3/4 full.Bake until golden:
Bake in an oven preheated to 350°F (175°C) for roughly 20 minutes, until the muffins are golden brown and firm to the touch.Cool in pan then rack:
Take the muffins out of the oven and allow them to cool in the pan for several minutes, then move them to a wire rack to cool fully.Serve and enjoy:
After cooling, take the muffins from the pan and serve. Relish the moist Donauwellen muffins with the melted Schokobon surprise inside.