Spread butter on one side of all the sliced bread. Flip the bread over and spread 1 tablespoon Nutella per bread. Set aside half of the sliced prepared bread.

Lay the sliced banana on the Nutella side of the remaining 4 slices of bread. Sprinkle 2 tablespoon marshmallows in an even layer. Stack with the reserved prepared bread to create a sandwich, buttered side facing up.

Heat a griddle or nonstick pan over medium heat. Lay sandwiches on the pan and heat until golden brown, about 2-3 minutes. Flip and heat on the other side until golden brown and the Nutella and marshmallows begin to melt. Remove from heat and serve.