In a saucepan, whisk together sugar, cocoa powder, cornstarch, and salt.

Slowly whisk in milk until smooth.

Cook over medium heat, stirring constantly, until mixture thickens and begins to bubble.

Reduce heat to low. In a small bowl, whisk egg yolks. Slowly add a few spoonfuls of hot chocolate mixture into yolks to temper.

Slowly whisk egg yolk mixture back into the saucepan.

Cook 2 more minutes, stirring constantly. Remove from heat; stir in butter and vanilla.

Pour filling into baked pie crust.

Chill in fridge 4–6 hours until set. Serve with whipped cream if desired.