1. In a heavy 2-quart saucepan, combine the 1 cup heavy whipping cream, chocolate pieces, butter and sugar. Cook over low heat, stirring constantly, till chocolate is melted, about 10 minutes. Remove pan from heat.
2. 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, till mixture is slightly thickened and almost bubbly – 3 to 5 minutes. Remove saucepan from heat. Stir in 3 tablespoons heavy whipping cream. Place saucepan in a bowl of ice water, stir constantly till mixture stiffens and becomes hard to stir (20 minutes).
3. Transfer chocolate mixture to a medium mixing bowl. Beat the cooled chocolate mixture with an electric mixer on medium to high speed until light fluffy (2 to 3 minutes). Spread filling in a baked pastry shell.
4. Cover and chill pie 4 to 5 hours or until set best served within 24 hours, but will last 3 to 4 days if stored properly. Serve with whipped cream and a garnish with chocolate curls.