In a heavy medium saucepan combine the 1 cup whipping cream, the chocolate pieces, butter, and sugar. Cook over low heat, stirring constantly, until chocolate is melted (about 10 minutes). Remove from heat. Gradually stir half of the hot mixture into beaten egg yolks. Return egg mixture to chocolate mixture in saucepan. Cook over medium-low heat, stirring constantly, until mixture is slightly thickened and begins to bubble (about 5 minutes). Remove from heat. (Mixture may appear to separate.) Stir in the 3 tablespoons creme de cacao or whipping cream. Place the saucepan in a bowl of ice water, stirring occasionally, until the mixture stiffens and becomes hard to stir (about 20 minutes). Transfer the chocolate mixture to a medium mixing bowl.