In a heavy 2-quart saucepan combine the 1 cup of whipping cream, chocolate pieces, butter and sugar. Cook over low heat, stirring constantly, until chocolate is melted. This should take about 10 minutes. Remove saucepan from heat. Gradually stir about half of the hot mixture into the beaten egg yolks. Return egg mixture to saucepan. Cook over medium-low heat, stirring constantly, until mixture is slightly thickened and nearly bubbly. This should take 3-5 minutes. Remove saucepan from heat. Mixture may appear to separate. Stir in creme de cacao or whipping cream. Place saucepan in a bowl of ice water; stir occasionally until mixture stiffens and becomes hard to stir (20 minutes). Transfer chocolate mixture to a medium mixing bowl. Beat the cooled chocolate mixture with an electric mixer on medium to high speed for 2-3 minutes or until light and fluffy. Spread filling in a baked pastry shell. Cover. Chill for about 5 hours or until set, or for up to 24 hours. At serving time top each piece with whipping cream and garnish with chocolate curls. Makes 10 servings.