Spread nutella on one slice of the bread and lay marshmallows in top. Then put your second slice of bread on top.
Lay a chosen amount of butter/margarine into a frying pan and let it sizle for 30 seconds. Put your sandwich (bread with marshmallows and nutella from step 1) in the fried butter into the pan and let it fry until it is golden.
Flip the bread over once o e side is gold and do the same with the other side. Serve with powdered sugar on top or just cut into quarters.