Freeze bananas for at least a few hours. If you're using them right away, you can freeze on a cookie sheet. If not, freeze in a covered freezer safe container.
When ready to use, chisel the solid chunk of frozen banana slices into a few pieces (if necessary) and throw it all in your food processor.
Pulse a few times to separate all the clumps and process until creamy, stopping a few times to scrape sides down with a spatula.
Add any extra ingredients in - chocolate, nuts, berries (optional)
You can eat it right away - it will be very creamy and custard like. Or you can throw it in the freezer and let it harden up a bit. I usually make this ahead of time, freeze it and let it sit at room temp for about 20 minutes before serving.