Place a skillet over medium heat. Spread butter evenly and completely over one side of each bread slice. Arrange the apple slices on the unbuttered side of one of the bread slices. Sprinkle the cinnamon over the apples. Place the cheese slice atop the apples. Top with the remaining bread slice with the buttered side facing outward. Lie gently into the skillet. Cook sandwich on both sides until golden brown, 2 to 3 minutes per side.