Peel the bananas and slice them into even 1-inch pieces, put them on a parchment-lined sheet pan and put in the freezer to harden for 8 hours minimum or overnight.
Once ready to make ice "cream," tip the frozen bananas into a blender, add the cinnamon and blitz together. Take the lid off and scrape down the sides with a rubber spatula, then pour in the milk and blend until smooth and creamy. Serve right away.