Our favorite one is at the RFC. It's called the volcano and it's big enough to feed all five of us. We always go to RFC at DTD our first night and get the volcano for dessert!
I also had this at california grill on January 10th it was very good with cherries on the side and a peppermint ice cream (the ice cream didn't seem to go with the dessert well but that's just my opinion)
If you're willing to go off site, Roy's (on Sand Lake Road) lava cake is legendary...pretty much the standard for which others are judged. Plus, the rest of their food is great, too.