Preheat the oven to 150 ° C (top / bottom heat). Beat the butter, sugar, vanilla sugar, eggs and the melted couverture until frothy. Stir in a pinch of salt and the milk. Mix the baking powder, pudding powder and flour, sieve over the mixture and stir in. Divide the batter into muffin molds and bake for 20 minutes.
For the buttercream, cook a pudding from 1/2 packet of pudding powder, 30 g of sugar and 250 ml of milk according to the instructions on the package. Melt 100 g couverture, mix with the pudding and let cool.
Mix the butter with the powdered sugar, then stir into the pudding until smooth. Squirt the buttercream onto the cooled tart with a piping nozzle and then decorate as desired.