Cut bananas into 1-inch chunks and place in large zip-close plastic freezer bag. Seal and freeze until frozen solid, 2–3 hours.
Place frozen bananas in food processor; puree, scraping down side of bowl with rubber spatula. Add vanilla extract and pulse to combine.
Serve immediately or, for firmer texture, transfer to container and freeze until ready to serve; allow to soften about 10 minutes at room temperature. Garnish with banana slices.
