Make sure the bananas are at least partially brown before chopping and freezing them.
Combine all ingredients in a blender, food processor, or Vita-mix. Process until completely smooth, adding a little milk of choice if your blender isn’t strong enough to handle the frozen banana. (If you’re not using a high-speed blender like a Vita-mix and don’t want to add any liquid, simply thaw the bananas a bit before blending.)
Either serve immediately as soft serve, or freeze for up to an hour before scooping out with an ice cream scoop for the classic ice-cream look. This tastes best the day it is made, but you can technically freeze leftovers for up to a few weeks and thaw before serving.