Step 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.

Step 2

Place egg yolks in a medium heatproof bowl. Gradually pour half of chocolate mixture into egg yolks, whisking constantly.

Step 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.

Step 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.