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