Place sugar, cocoa powder, cornstarch, and salt into a medium sized saucepan.
Whisk until no lumps remain.
Whisk in heavy cream and milk.
Place over medium heat and bring to a light boil (it’ll look like slow, thick bubbles), whisking occasionally.
Once bubbling, whisk constantly (it’ll get thick quick) and cook for 1 minute, the mixture should be thick enough to coat the back of a metal spoon.
Remove from heat, and whisk in butter and vanilla.
Let cool for 5 minutes, then re-whisk (a film will form) and pour into crust.
Cover with plastic (the plastic will stick to the top of the pudding - that's good, this keeps a film from forming) and chill until the pudding has set, at least 30 minutes to an hour.
Slice and serve with optional garnishes.
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