In the top of a double boiler over hot water, melt 4 squares of bitter chocolate with 1 tablespoon of butter. Combine 1 cup each of sugar and light cream. Add them to the melted chocolate. Place the sauce over direct heat and cook, stirring sauce, until it reaches the boiling point. Reduce heat to very low and continue to cook until it thickens slightly. Stir in 1 teaspoon of vanilla.