Melt the chocolate and butter in separate pans, let them cool.
Preheat the oven to 180°C.
In a bowl, mix the chestnut purée with the milk until creamy. Add the beaten eggs, then the cooled butter and finally the rum, if used.
In a large mixing bowl sieve the flour and baking powder together, mix in the sugar, then gently stir in the chestnut paste, jam and melted chocolate.
Line a muffin tin with paper cases or use silicon muffin cases. Fill them with the batter and bake for about 25 minutes.
Let them cool on a wire rack and decorate them to your liking.