Have your grill going at a medium heat. If you have a temperature gauge, aim for a range of 300°F to 350°F. Have rack set at its most indirect position.
Lay bananas on counter and allow them to naturally flop to one side; they need to be sturdy to hold the fillings. Use a paring knife to make a slit going almost all the way down through the skin and flesh, but leave about 1-inch solid on either end. Gently pry the slit open revealing a cavity. Divide the marshmallows and morsels equally among the bananas stuffing the openings.
Place bananas directly on indirect grill grate, close grill and bake until marshmallows and chocolate have melted and the tops of the marshmallows are beginning to brown. This can take anywhere from 3 to 10 minutes depending on the grill heat. Just bake until done. Immediately place on plates, top with small scoops of vanilla ice cream, sprinkle graham cracker crumbs on top and serve with spoons.
Recipe by Bakepedia at https://www.bakepedia.com/smore-banana-boat-sundaes/