Spray the grill rack with nonstick, nonflammable cooking spray. Preheat the grill to medium heat.
In a small bowl, add the strawberries, sugar, and warm water. Set aside. The sugar will dissolve and create a syrup.
Spread 1 side of half of the bread slices with about 1 tablespoon marshmallow creme. Spread 1 side of the remaining bread slices with about 1 tablespoon chocolate hazelnut spread. Place sliced bananas on top. Top each marshmallow creme bread slice with a chocolate hazelnut bread slice to make a sandwich. Brush the outsides of the sandwiches evenly with the melted butter.
Grill the sandwiches until the bread is toasted and the marshmallow creme has melted, about 2 minutes per side. Drizzle with the strawberries and their syrup on top and serve immediately.