Black bananas work best because a lot of the starch has been converted to natural fruit sugars. If you've got any going off in your fruit bowl simply peel them and pop them whole in a tub in the freezer until you're in the mood for ice cream.
One banana = one serving
Remove your banana from the freezer about 15-20 minutes before you start making the ice-cream unless you have a really powerful food processor.
It takes up to two minutes to fully blend your banana.
Serve immediately after blending.
As you've already partially defrosted your banana I wouldn't recommend refreezing your ice-cream.