Stir together the sugar, cocoa, cornstarch, and salt in a saucepan.
Add the vanilla to the milk then slowly add in the milk to the chocolate mixture while continually whisking.
Cook over medium heat, whisking constantly, until mixture starts to boil; boil and stir for 1 minute.
Remove from the heat, stir in butter.
Pour into individual dessert dishes.
To avoid a skin from forming on top, press saran wrap directly onto the surface; serve warm or refrigerator for 2 hours prior to serving cold.
Serve plain or with whipped cream. Enjoy.