On a lightly floured surface, use your hands to slightly flatten Cocoa Pastry. Roll dough from center to edge, forming a circle about 12 inches in diameter. To transfer pastry, wrap it around the rolling pin. Unroll pastry into a 9-inch pie plate. Ease pastry into pie plate, being careful not to stretch pastry. Trim pastry to 1/2 inch beyond edge of pie plate. Fold under extra pastry. Crimp edge as desired.
Don't prick pastry. Line with a double thickness of foil. Bake in a 450 degree F oven for 8 minutes. Remove foil. Bake for 5 to 6 minutes more or until pastry is set and dry. Cool on a wire rack.
For filling, in a medium saucepan, combine granulated sugar and cornstarch. Gradually stir in milk. Cook and stir over medium-high heat until mixture is thickened and bubbly. Reduce heat; cook and stir for 2 minutes more.
Gradually stir about 1 cup of the hot filling into egg yolks. Return all of the mixture to the saucepan; bring to a gentle boil. Cook for 2 minutes more. Remove from heat. Stir in 1-1/2 teaspoons vanilla.
Add chocolate pieces and butter to the filling; stir gently until chocolate and butter are just melted. Spoon into baked pastry shell. Place a piece of plastic wrap on surface of the filling. Cool for 1 hour. Chill in refrigerator for at least 2 hours or up to 24 hours.
Before serving, in a chilled bowl, combine whipping cream, powdered sugar, cocoa powder, and the 1/2 teaspoon vanilla. Using chilled beaters, beat with an electric mixer on medium speed until soft peaks form (tips curl). Dollop on pie just before serving. Garnish pie with chocolate curls or chocolate-flavored sprinkles, if you like. Makes 10 servings.
In a bowl, stir together flour, cocoa powder, sugar, and salt. Using a pastry blender, cut in butter until pieces are peasize. Sprinkle 1 tablespoon of the cold water over part of the mixture; gently toss with a fork. Push moistened dough to side of bowl. Repeat moistening dough, using 1 tablespoon of the cold water at a time, until all the dough is moistened. Form dough into a ball.