For the chocolate cupcakes, preheat the oven to 180°C degrees. For the batter, melt the chocolate in a bain-marie. Beat the eggs, butter and sugar until fluffy, then mix in the cooled melted chocolate and the buttermilk. Mix flour with baking powder and salt and fold in. Pour the batter into muffin or cupcake tins lined with paper cups and bake for about 30 minutes at 180°C degrees. Then leave to cool.
For the cream, mix 6 tablespoons of the milk with the cornstarch. Bring the remaining milk to the boil with the sugar and vanilla sugar. Add the cornstarch and bring to the boil once briefly. Refrigerate this custard for about 1 hour. Stir from time to time so that a skin does not form. Beat the butter until fluffy and gradually add the pudding mixture and the jam. Chill the custard for an hour before piping it onto the cooled chocolate cupcakes.