1-1/3 cups chocolate wafer crumbs (from about 26 cookies such as Nabisco Famous Chocolate Wafers)
5 Tbsp unsalted butter, melted
1/4 cup sugar
2/3 cup sugar
1/4 cup cornstarch
1/2 tsp salt
4 large egg yolks
3 cups whole milk
5 oz fine-quality bittersweet chocolate (not unsweetened), melted
2 oz unsweetened chocolate, melted
2 Tbsp unsalted butter, softened
1 tsp vanilla
3/4 cup chilled heavy cream
1 Tbsp sugar
Put oven rack in middle position and preheat oven to 350°F.
Stir together crumbs, butter, and sugar and press on bottom and up side of a 9-inch pie plate (1-quart capacity). Bake until crisp, about 15 minutes, and cool on a rack.
Whisk together sugar, cornstarch, salt, and yolks in a 3-quart heavy saucepan until combined well, then add milk in a stream, whisking. Bring to a boil over moderate heat, whisking, then reduce heat and simmer, whisking, 1 minute (filling will be thick).
Force filling through a fine-mesh sieve into a bowl, then whisk in chocolates, butter, and vanilla. Cover surface of filling with a buttered round of wax paper and cool completely, about 2 hours.
Spoon filling into crust and chill pie, loosely covered, at least 6 hours.
Just before serving, beat cream with sugar in a bowl using an electric mixer until it just holds stiff peaks, then spoon on top of pie.