Peel the bananas and cut them into slices. Pop the slices into a food storage bag and freezer for at least 4 hours or overnight, until solid.
Pour the frozen bananas into a food processor or heavy-duty blender. Pulse until crumbs forms, then keep pulsing until the bananas start to smooth out. Scrap down the side of the bowl with a rubber spatula.
Process the bananas until completely smooth, scraping the bowl as needed. After about 10 minutes, the bananas should resemble soft-serve ice cream.
Serve immediately as soft-serve. For a firmer, scoopable ice cream transfer to a small loaf pan and freezer for 2 hours.
Notes
You can make berry banana ice cream using 3 bananas (sliced and frozen) + 2 cups of frozen berries.
Make chocolate peanut butter banana ice cream by adding 1/4 cup creamy peanut butter and 1/4 cup sweetened cocoa powder to 4 bananas (sliced and frozen).