1 In a heavy 2-quart saucepan combine the 1 cup whipping cream, the chocolate pieces, butter, and sugar. Cook over low heat, stirring constantly, until chocolate is melted (this should take about 10 minutes). Remove from heat. Gradually stir about half of the hot mixture into beaten egg yolks. Return egg mixture to mixture in saucepan. Cook over medium-low heat, stirring constantly, until mixture is slightly thickened and near bubbly (this should take 3 to 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; stir occasionally until the mixture stiffens and becomes hard to stir (this should take about 20 minutes). Transfer the chocolate mixture to a medium mixing bowl.
2 Beat cooled chocolate mixture with an electric mixer on medium to high speed for 2 to 3 minutes or until light and fluffy. Spread filling in the baked piecrust. Cover and refrigerate pie for 5 to 24 hours.