1. Whisk together sugar, cornstarch and cocoa in a medium saucepan
2. Put on the stove, over medium heat, and slowly whisk in heavy cream, milk, vanilla extract and a pinch of salt
3. Increase heat to medium-high and cook, whisking constantly, until mixture thickens, about 5-7 minutes
4. Whisk in chocolate, until melted, 1-2 minutes
5. Remove from heat and whisk in butter until completely melted (this step is optional, but it gives the pudding a nice shine)
6. Working quickly, pour or ladle pudding into a serving dish and cover tightly with plastic wrap, placing wrap directly on the surface of pudding to prevent it from getting a skin. Refrigerate until set, at least 1 hour, before serving

Serving Suggestion: Garnish with whipped cream and/or grated or shaved semi-sweet chocolate!