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1
Blend together flour, butter, shortening, and salt in a bowl with your fingertips or a pastry blender (or pulse in a food processor) just until mixture resembles coarse meal with some roughly pea-size butter lumps. Drizzle 2 tablespoons ice water evenly over mixture and gently stir with a fork (or pulse) until incorporated.
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2
Squeeze a small handful of dough: If dough doesn't hold together, add more ice water, 1 tablespoon at a time, stirring until incorporated. (Do not overwork dough or pastry will be tough.)
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3
Turn out dough onto a lightly floured surface and divide into 4 portions. With heel of your hand, smear each portion once or twice in a forward motion to help distribute fat. Gather all of dough together, with a pastry scraper if you have one, and form into a 5-inch disk. Chill, wrapped in plastic wrap, until firm, at least 1 hour.
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4
Roll out dough on a lightly floured surface with a lightly floured rolling pin into an 11-inch round, then fit into a 9-inch pie plate. Trim edge, leaving a 1/2-inch overhang, then fold overhang under and crimp edge decoratively. Prick bottom and side of shell all over with a fork, then chill shell 30 minutes.
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5
While shell chills, preheat oven to 375u00b0F with a baking sheet on middle rack.
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6
Line shell with foil and fill with pie weights. Bake on baking sheet until pastry is set and edge is pale golden, about 25 minutes. Carefully remove weights and foil, then bake shell on baking sheet until pale golden all over, 15 to 20 minutes more. Cool shell.
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7
Whisk together cornstarch, 1/3 cup sugar, cocoa powder, and salt in a 2-quart heavy saucepan, then gradually whisk in milk. Bring to a boil over medium heat, whisking constantly, then boil, whisking, 2 minutes (mixture will thicken). Remove from heat and whisk in chocolate and vanilla until smooth.
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8
Pour filling into cooled shell and chill, its surface covered with wax paper (if you want to prevent a skin from forming), until cold, at least 2 hours.
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9
Just before serving, beat cream with remaining 2 tablespoons sugar until it just holds soft peaks. Spoon onto pie.
