Preheat your grill to medium heat or your oven to 350°F.
Keeping the peel on, slice each banana lengthwise down the middle, cutting about three-quarters of the way through but not cutting through the bottom peel.
Gently open each banana and spread 2 tablespoons of Nutella inside, then sprinkle with mini marshmallows and chocolate chips.
Wrap each banana in aluminum foil and grill or bake for 8 to 10 minutes, until the banana is soft and the fillings are melted, then serve warm with vanilla ice cream if desired.