1. To make the crust: put oven rack in middle position and preheat oven to 350 degrees.
2. Stir together crumbs, butter, and sugar and press on bottom and up side of a 9-inch pie plat (1 quart capacity).
3. Bake until crisp, about 15 minutes, and cool on a rack.
4. To make the filling: Whisk together sugar, cornstarch, salt, and yolks in a 3-quart heavy saucepan until combined well, then add milk in a stream, whisking.
5. Bring to a boil over moderate heat, whisking, then reduce heat and simmer, whisking, 1 minutes (filling will be thick).
6. Force filling through a fine-mesh sieve into a bowl, then whisk in chocolates, butter, and vanilla.
7. Cover surface of filling with a buttered round of wax paper and cool completely, about 2 hours.
8. Spoon filling into crust and chill pie, loosely covered, at least 6 hours.
9. To make the 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.
