In a medium pan, whisk together the sugar, cocoa, cornstarch and salt
Whisk the milk in to the mixture and heat to boiling, stirring constantly
Once it has started to boil, continue to cook and stir for another 2 minutes
Remove from heat and stir in butter and vanilla
Transfer to serving dishes and chill in the refrigerator (pudding will thicken as it cools)
Serve and enjoy!