Preheat the oven to 200 degrees top / bottom heat.
Mix the butter with sugar and eggs until frothy. Sieve the flour, cocoa, baking powder and gingerbread spice on top and gradually add the milk and rum while stirring.
Place the dough on a baking sheet lined with baking paper, distribute it evenly and smooth it out. Bake on the middle rack for about 30 minutes. The baking time varies depending on the oven and the size of the baking tray.
Let the cake cool, cover with melted couverture and decorate with colorful sugar sprinkles as desired.