1. In a medium, heavy bottomed saucepan, whisk together sugar, cornstarch, cocoa, and salt. Combine half-and-half, milk, and vanilla in a bowl. Whisk 1 cup of the liquid with the dry ingredients until cornstarch is completely dissolved. Whisk in remaining liquid.

2. Place saucepan over medium heat. Cook, whisking constantly, until mixture comes to a boil and thickens, about 4-5 minutes. Add chocolate, and cook, whisking until the chocolate is melted, about 1 minute. Remove from heat and whisk in butter until melted. Pour pudding into six dessert cups. It can be eaten warm (best) or refrigerate until completely set, about 1 hour.