Molten tire at NASCAR NSG, IMHO
Warm chocolate banana gooey cake at Mythos is FABULOUS. The shots are boring.
Good sundaes at Hard Rock, also one called the San Francisco Foggie at Lombard's in USF.
BUT, for the absolute best dessert you'll have in Orlando (maybe ever), you'll have to leave Universal and take a short drive up Kirkman to
The Dessert Lady. Hard to find as it's tucked away in a strip mall on the corner of Kirkman and Conroy (turn left, then left again), but worth the effort.
Everything's good there, but I am specially partial to the key lime cake. They also serve very good wines.