Gordon Ramsey's chocolate fondants (like chocolate cake, but with a runny middle) would be a GREAT dessert for this. Fondants are notoriously hard to cook BUT GR's seem to come out right EVERY time. They are dead easy to make - the only hard bit is deciding when to take them out of the oven. I've even tried overcooking them just to see what they'd be like, and even overcooked they are still a DELICIOUS chocolate cake (rather than a fondant). So don't tell your guest what you're making for dessert until it's cooked, and if you get it wrong, just tell them the dessert is chocolate CAKE.