Peel ripe bananas, slice into 1-inch rounds, and freeze in a single layer on a parchment-lined baking sheet for at least 2 hours or overnight. Transfer to a zip-top bag once frozen.
Add frozen banana pieces to a high-speed blender or food processor. Blend on high, scraping down the sides as needed. Add milk 1 tablespoon at a time only if needed. Blend until completely silky and smooth - about 60–90 seconds.
Add your chosen flavor add-ins and pulse briefly to combine. Taste and adjust as desired.
Serve immediately for soft-serve, or transfer to a container and freeze for 1–2 hours for a scoopable texture. Let sit at room temperature for 5 minutes before scooping.