Spread butter on one side of each bread slice. Arrange cheese and apple slices on the unbuttered side of two of the bread slices. Top with remaining two bread slices, buttered sides up (sandwiches will be full).
Heat a griddle or flat-surfaced skillet over medium heat. Place the sandwiches on the griddle or in the skillet and cook until golden brown on one side, about 4 minutes. Turn and cook until second side is golden brown and crispy, 3 to 4 minutes more.