I went to a Mastro's in Chicago with my brother. I loved their butter cake so much that after seeing a show in Vegas, I took my wife to experience the greatness that is Mastro's Butter Cake.
If you haven't tried it and you are on death row, this should be your final meal (for dessert). If you are not on death row or in prison, you should definitely make a trip and try it out. The rest of their food is really good as well, but we came for THE dessert.
I can't describe it, except it is scrumtralecent. I envy anyone who lives within an hour of Mastro's. If you live within an hour and haven't tried it yet you should be deported... to Idaho.