Peel bananas, place in large zipper-lock bag, and press out excess air. Freeze bananas until solid, at least 8 hours.
Let bananas sit at room temperature to soften slightly, about 15 minutes. Slice into ½-inch-thick rounds and place in food processor. Add coconut milk, vanilla, lemon juice, salt, and cinnamon and process until smooth, about 5 minutes, scraping down sides of bowl as needed.
Transfer mixture to airtight container and freeze until firm, at least 2 hours or up to 5 days. Serve.
