Frozen bananas are a southern california thing. They are famous on Balboa Island. They are a banana, frozen, and then covered in a chocolate shell. You can usually get them with toppings on them too. They are pretty much amazing, but extremely hard to come by outside of SoCal.
If you're ever in Princeton, The Bent Spoon (best ice cream in NJ) has this machine that takes frozen banana (nothing else) and extrudes it into this amazing creamy gelatto type texture. Has anyone heard of anyplace that does this in Manhattan?