Melt the chocolate slowly in a double boiler over simmering water. Remove from heat and beat egg yolks into chocolate with electric mixer.
Beat in rum, orange liqueur, or vanilla extract, if desired. In a separate bowl, beat egg whites to medium-stiff peaks. Stir a large spoonful of egg whites into the chocolate mixture to loosen. Then gently fold the remaining egg whites into the chocolate. Pour into serving glasses and chill in refrigerator until set, 4 to 5 hours. Will keep up to four days in refrigerator.