Heat hazelnut spread, so that it lightly coats the back of a spoon. Spread all slices of bread with a thin layer of hazelnut spread. Apply banana slices to 4 pieces of bread and marshmallows to the remaining 4 pieces. Take 1 slice of bread with banana, and 1 with marshmallows and press them together, making a sandwich. Repeat. Apply a light layer of butter to each side of the sandwiches.
Place each sandwich on the pre-heated grill for 2 minutes per side. Once cooked, cut each sandwich in half on the diagonal. Put sandwiches on a platter and dust with powdered sugar.