Whisk sugar, cocoa powder, corn starch, and salt together in a medium sized saucepan. Slowly add milk while whisking.
Cook over medium heat while continually whisking. You will notice it starting to thicken up right before it starts to bubble. Once boiling, let it cook for a minute while still whisking.
Remove from heat and whisk in butter and vanilla.
Pour into serving dishes or a bowl and cover with plastic wrap. Press plastic wrap into the pudding. Refrigerate until cold, about 2 hours.
Notes
Serving Suggestion
Serve with whipped cream for an extra delicious treat.