Butter one slice of bread with half the butter. Place buttered side down in cold skillet.
Remove wrapper from chocolate bar; break into sections. Evenly space chocolate bar pieces on unbuttered side of bread. Layer with graham cracker, breaking to fit. Top with marshmallow halves and second slice of bread. Butter top of sandwich with remaining butter.
Heat over medium low heat until bottom of sandwich is golden. Carefully flip sandwich; heat until bottom is golden. Remove from skillet; cool slightly. Cut and serve. Makes one sandwich.