In a small bowl combine butter and cinnamon. Set aside.
Lightly spread one side of a bread slice with butter mixture. Flip, and top with marshmallows. Spread one side of remaining bread slice with peanut butter. Place on top of marshmallows, peanut butter side down and spread remaining butter mixture on top.
Preheat a medium skillet over medium heat. Carefully place sandwich in the skillet. Cook sandwich for 6 minutes or until bread is golden and marshmallow candies are softened, turning once halfway through cooking. Cut sandwich in half or into quarters. Serve immediately.