Chocolate cream pie

Always a bestseller at parties. I get asked to make this loads of times!

For crust:
1 1/2 cups chocolate wafer crumbs (eg Nabisco chocolate wafers)
5 tbsp melted unsalted butter
1/4 cup granulated sugar

For filling
2/3 cup sugar
1/4 cup cornstarch
1/2 tsp salt
4 large egg yolks
3 cups whole milk
5 oz bittersweet chocolate, melted
2 oz unsweetened chocolate, melted
2 tbsp unsalted butter, softened
1 tsp vanilla

For topping
3/4 cup heavy cream
1 tbsp sugar

Make crust: 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 (1 quart) pie plate and bake about 15 minutes, until crisp. Cool on a rack.

Make filling: 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.

Make topping: 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. Make sure cream, bowl and beaters are very cold--refrigerate a few minutes if necessary.Pie can be chilled up to 1 day without the topping.