In a saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt.
Add yolks and whisk.
Slowly pour in the milk, whisking constantly.
Heat this over medium heat but make sure you keep whisking!
Boil 30 seconds and then pour into a bowl.
Whisk in the butter, chocolate and vanilla.
Let cool 15 mins at room temp and press a piece of plastic wrap on top-chill 3 hours.
Scoop into dishes and top with whipped cream and chocolate shavings!