Whisk together cornstarch, cocoa and salt in a heavy saucepan. Slowly whisk in half-and-half. Turn heat to medium and cook, whisking frequently, until mixture thickens, 8 to 10 minutes.
Remove from heat and whisk in chocolate-hazelnut spread, butter and vanilla until smooth. Divide among 6 ramekins or custard cups. Serve warm, or cover, pressing plastic wrap onto surface, and refrigerate for up to 3 days. Serve with whipped cream, if desired.