In a heavy, medium-size saucepan over medium heat, bring the chocolate and cream to a boil, stirring often. Reduce the heat to simmering and simmer until the sauce is thickened enough to heavily coat a spoon, about 3 minutes. Remove from the heat and stir in the rum. Serve the sauce warm or at room temperature. The sauce can be stored, covered, in the refrigerator for up to 1 week. Reheat the sauce in a double boiler over simmering water.