In a medium heavy pan, whisk sugar, cocoa, cornstarch and salt. Slowly whisk in the milk and cream and vanilla bean extract (if using). Make sure there are no lumps.
Slowly bring the mixture to a boil over medium heat while whisking constantly.
Lower the heat to a simmer cover and simmer gently for 8~10 minutes or until the pudding begins to thicken. Remove from the heat and whisk in the butter and vanilla extract.
Let the pudding cool completely.