Slice the bananas into 1/2"-1" pieces. Place the banana pieces in the freezer for 1-2 hours. (Do not try this with bananas that have been in the freezer overnight. Learn from my mistakes!) In a food processor or a very powerful blender, puree the frozen banana chunks. Scrape down the sides of the bowl as needed and continue blending until smooth. Once the bananas have turned into ice cream, enjoy as is or add the peanut butter for a truly amazing treat. This is an excellent soft serve ice cream straight out of the food processor.
You can freeze it further, simply by placing the ice cream in your freezer once it is smooth. This will result in a firmer, yet still scoop-able ice cream within a few hours.