Pour the milk and cream into a saucepan and bring to a simmer over medium heat.
While the milk heats, whisk the yolks in a large bowl until they’re light. Add the sugar, cornstarch, cocoa, and a pinch of salt, and whisk. This will be pretty stiff.
Once the milk is simmering, pour about a cup of it into the yolks, whisking constantly. Pour in the rest of the milk, whisk, then pour it all back into the saucepan. Return to the heat and whisk constantly until the mixture thickens and comes just to a boil—one or two big bubbles. Turn off the heat and add the chocolate. Whisk to melt the chocolate, then get out a rubber scraper and get into the corners of the pan, making sure you don’t have any light streaks left. Whisk in the butter and vanilla until the butter melts.
Pour the pudding into a large bowl, or into six small ones—custard cups, footed dessert bowls, that kind of thing. Chill for at least an hour before serving. If you have some aversion to skin, press plastic wrap in the surface of the pudding before chilling.
Beverage pairing: Banfi Rosa Regale, Italy. Brachetto is the grape, and the wine is a sweet, gently sparkling dark rosé with irresistible notes of strawberry, raspberry, and roses. It’s a good match with any form of chocolate, but particularly a pudding like this one, as the modest carbonation makes an appealing contrast with the creaminess of the dessert.
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