If your bananas are not already frozen, break them up into small pieces and add to a freezer safe bag or container. Freeze for at least 12 hours.
Once frozen, add the frozen bananas, cocoa powder, nut butter and vanilla to a high speed blender or food processor with an S blade.
A high speed blender (like a vitamix) is necessary, a regular one won't work! If using a food processor, you'll need about 2 tablespoons of non-dairy milk to help the blending! Blend on high, using the tamper tool to push down as needed. It will be loud and seem like it's not working at first, but keep blending and eventually you will end up with a soft serve like consistency.
You can add more milk as needed, but only add enough to start the blending process. If using a food processor, stop to scrape down the sides as needed.
Taste and adjust flavors as desired. If using the oreos, add them in now and blend for just a few seconds to combine and break up.
Serve as is, like soft serve, or transfer to a loaf pan lined with wax or parchment paper and freeze for 2-3 hours to harden.
Longer than that and you will need to let it thaw for 10-15 minutes before scooping, or microwave for about 1 minute or until soft enough to scoop.
Serve with vegan caramel, magic shell, chocolate chips, crushed oreos or enjoy as is!