Break chocolate into pieces. Melt with butter in a hot water bath. Crumble the cake with your hands. Add cocoa and gingerbread spice. First stir in chocolate butter with the whisk of the mixer, then rum bit by bit - the mixture should be malleable.
Form about 40 cherry-sized balls from the mass. Chill for at least 30 minutes.
Chop white chocolate coating. Melt in a hot water bath. Cut the cherries into eighths. Pour the chocolate coating into a freezer bag. Cut off a small corner. Dribble over the truffles. Place 1 cherry box on each.
Cool for at least 1 hour. Remove from the fridge about 30 minutes before serving.