Preheat oven to 300ºF. Slit each banana lengthwise through 1 side of peel, taking care to avoid cutting all the way through banana. Stuff each slit with chocolate chips and marshmallows.
Wrap each banana in aluminum foil, place upright on a baking sheet and bake until chocolate and marshmallows have melted, 10 to 15 minutes. Unwrap and serve immediately.