Roughly chop the chocolate. Melt 125 g butter and chocolate in a saucepan over low heat while stirring, remove from the heat and pour into a mixing bowl. Add sugar, vanillin sugar, salt, eggs, flour and baking powder.
Mix to a smooth dough with the whisk of the hand mixer. Place 2 paper baking cups in each of the recesses of a muffin tray (12 recesses). Pour in the dough and bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 20 minutes.
Take the muffins out of the oven and let them cool on a cake rack. Wash the blueberries and drain them on kitchen paper. Just before serving, mix mascarpone, cream cheese, cream and icing sugar well.
Fold 150 g blueberries into the mascarpone cream and spread on the muffins using a spoon. Decorate with remaining blueberries.