First freeze your bananas either overnight or for at least 2-3 hours. Make sure to peel them before freezing and either put them in a freeze safe bag or just on a pan lined with parchment paper.
Once bananas are frozen, add all the ingredients (except for cookies and any additional toppings) to a high speed blender or food processor. Blend until smooth and creamy. NOTE: A splash of non-dairy milk might be needed to help it blend better.
Assemble ice cream sandwiches. If you would like chocolate chips on the side, pour them on a plate and dip the sides of the sandwiches in them before freezing. Once you assemble one, immediately put into freezer for at least an hour before serving.
NOTE: A food processor or high speed blender works best. If the nice cream is too creamy, you might need to let it sit in the freezer for 20 minutes or so to firm up just a bit. You don't want it too frozen or too creamy because it will be hard to assemble.