Freeze the bananas ahead. Peel ripe bananas and slice into 1/2-inch coins. Spread on a parchment-lined baking sheet and freeze for at least 2 hours, or overnight.
Prepare your blender. Remove frozen banana slices from freezer and let sit at room temperature for 2-3 minutes to slightly soften. This prevents motor strain and creates smoother texture.
Add ingredients to blender. Place frozen banana slices in a high-powered blender or food processor. Add 1 tablespoon milk and vanilla extract if using. If making a flavored version, add those ingredients now.
Blend in stages. Start blending on low, using a tamper if you have one to push bananas toward the blade. The mixture will go through stages: crumbly, then chunky, then suddenly smooth and creamy like soft-serve. This takes 2-4 minutes.
Add more liquid if needed. If the mixture won't blend smoothly, add milk 1 tablespoon at a time. Too much liquid makes it soupy, so add conservatively.
Check consistency. The nice cream is ready when it's smooth, creamy, and resembles soft-serve ice cream. It should hold soft peaks when you scoop it.
Serve immediately or freeze. For soft-serve texture, serve right away. For firmer ice cream texture, transfer to a container and freeze for 30-60 minutes, then scoop and serve with desired toppings.