Whisk together sugar, cornstarch, cocoa powder and a pinch of salt in a heavy saucepan, then whisk in milk and cream. Bring to a boil over moderately high heat, whisking constantly, then boil, whisking, 2 minutes (mixture should be thick). Remove from heat. Whisk in chocolate and vanilla until smooth. Transfer to a bowl and chill at least 2 hours, until cold (surface may be covered with wax paper to prevent skin from forming, or, if you’re like me who tends to buy the bare necessities, plastic wrap). Can be chilled, covered, up to 3 days.

Serve with lightly sweetened whipped cream, or plain.