Cream butter, vanilla sugar, salt and sugar with the whisk of the hand mixer. Stir in the eggs one after the other. Mix flour and baking powder and stir in alternately with the yoghurt. Stir in the candy briefly. Put the dough into a well greased fat pan of the oven (32x39 cm), smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for about 25 minutes.
Take out of the oven, sprinkle with rum, place on a grid and let it cool down. Spread plum puree evenly on the cooled cake. Put the cream and sour cream in a mixing bowl and whisk until stiff with the whisks of the hand mixer. Spread loosely on the cake base. Cut into 24 pieces and sprinkle with some candy. Decorate with lemon balm