Mix Nutella and whipped cream cheese together until combined.
Butter one side of each slice of bread On opposite side of one slice of bread spread Nutella and cream cheese mixture. On other slice of bread spread on marshmallow fluff. Add bananas to one side.
Heat a nonstick skillet to medium to medium high heat. Melt 1 tablespoon butter in bottom of pan.
Place one side of sandwich in the pan butter side down and top with other side butter side up. Grill until each side is golden brown.