Mix the butter, sugar, vanilla sugar and salt until creamy. Stir in the eggs one at a time. Mix the flour, baking powder and cinnamon, alternately stir in with the milk.
Halve the plums, stone them and cut into fine cubes. Distribute the batter evenly in the paper cases in the muffin tin.
Bake in a hot oven at 175 ° C for approx. 20 minutes.