Fill up sheet pans with Graham crackers. Spread each with a thin layer of Nutella. Put 6-7 mini marshmallows on top of each. Put under the broiler for about 2 minutes. Watch to make sure they get golden brown. Take them out and let them cool slightly. Make sandwiches by pressing two together. Eat right that moment or let them cool completely and wrap each with plastic wrap. Keep cool until serving time but do not chill them in the fridge or the cracker will get soggy.