Prepare Cocoa Pastry. On a lightly floured surface, flatten dough with your hands. Roll dough from center to edges, forming a circle about 12 inches in diameter. Wrap pastry around rolling pin. Unroll 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. Flute edge. 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 till pastry is set and dry. Cool on a wire rack.
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.
Beat the egg yolks lightly with a fork. Gradually stir about 1 cup of the hot filling into yolks. Return all of the mixture to the saucepan; bring to a gentle boil. Cook 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 for 2 to 24 hours.
Before serving, in a chilled mixing bowl, combine the whipping cream, powdered sugar, cocoa powder, and the 1/2 teaspoon vanilla. Beat with chilled beaters of an electric mixer on medium speed until soft peaks form. Dollop on pie just before serving. If you like, garnish pie with chocolate curls or chocolate-flavored sprinkles. Makes 10 servings.
In a mixing bowl, stir together flour, cocoa powder, sugar, and salt. Cut in butter until pieces are the size of small peas. Using cold water, sprinkle 1 tablespoon of the water over part of the mixture; gently toss with a fork. Push to side of bowl. Repeat until all is moistened. Form dough into a ball.