In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt. Over medium heat, slowly add the milk, whisking constantly. Bring the mixture to a boil and continue to whisk for an additional minute. The mixture will be thick at this point. Remove from heat and stir in the vanilla and butter until melted.
Pour into individual serving dishes if desired. Cover with plastic wrap, pressing it directly onto the pudding to avoid forming a “skin”, and chill for at least one hour.
