I keep making this and looking up the recipe each time is a pain in the buns. So here it is:
2/3 cup sugar
1/4 cup cornstarch
1/2 teaspoon salt
4 large egg yolks
3 cups whole milk
5 oz fine-quality bittersweet chocolate (not unsweetened), melted
2 oz unsweetened chocolate, melted
2 tablespoons unsalted butter, softened
1 teaspoon vanilla
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.