2 cups milk
1/2 cup sugar
1/4 cup unsweetened cocoa powder
3 tablespoons cornstarch
1/4 teaspoon salt
2 tablespoons butter
1 teaspoon vanilla extract
Whisk together sugar, cocoa powder, cornstarch, and salt in a saucepan
Gradually whisk in milk until smooth
Cook over medium heat, stirring constantly, until mixture thickens and starts to bubble
Boil gently for 1 minute while stirring
Remove from heat
Stir in butter and vanilla extract
Pour into serving dishes
Chill until set
Serve cold