Slice the bananas before you freeze them, in a tupperware container or a bag. This will allow it to move into the blades easier. Place them in a food processor or high powered blender and blend them. I always use a food processor for nice cream because it works better. It will take some time, and you might need to move the meat down into the blades, or use a plunger if you are using a blender (high powered blenders only will do this). It will begin to move, form, and then turn into a whip.
Add the cacao nibs last and let them break up. Serve immediately.