1 (9 inch) pie crust, baked
1 1/2 cups white sugar
1/3 cup cornstarch
1/2 teaspoon salt
3 cups milk
3/4 cup semisweet chocolate chips
2 (1 ounce) squares unsweetened chocolate, chopped
4 egg yolks, beaten
1 tablespoon vanilla extract
1 Combine sugar, cornstarch, and salt in a 2-quart saucepan. Stir in milk gradually. Add chocolate chips and unsweetened chocolate. Place over medium heat, stirring constantly, until mixture thickens and boils. Boil and stir 1 minute.
2 Place egg yolks in a medium heatproof bowl. Gradually pour half of chocolate mixture into egg yolks, whisking constantly.
3 Whisk egg yolk mixture back into mixture in saucepan. Place over medium heat and bring back to a boil, stirring constantly. Boil and stir 1 minute. Remove from heat; stir in vanilla extract.
4 Pour mixture into baked pie shell. Press a layer of plastic wrap onto filling. Refrigerate at least 4 hours but no longer than 48 hours. Remove plastic wrap before serving and top with whipped topping.