This is sweet and oh so warm and gooey. Try to let them cool off, grab a spoon, and dig in.
Line a skillet with aluminum foil. Add crushed graham crackers and butter and mix to combine. Press lightly in bottom of skillet. Pour condensed milk over cracker crumbs, add the chocolate pieces, and cover with marshmallows. Place on grill and cover. Cook until marshmallows are nicely browned and chocolate is melted, about 10 minutes.