You don't need a box! Can be doubled. I have also substituted low fat milk and half and half. Feel free to experiment with milk substitutes if you like. The corn starch really "sets" this pudding.
In a heavy-bottomed saucepan, stir together granulated sugar, cocoa, cornstarch and salt Slowly pour in 1 ½ cups milk, stirring or whisking to create a lump-free mixture Stir in 1 cup heavy cream.
Place saucepan over medium heat and, stirring gently and continuously scraping bottom and sides of pan, bring mixture to a boil (boil 1 minute) Remove from heat.
Stir in 1 teaspoon vanilla extract.
Pour pudding into stemmed glasses or custard cups or other container.