Heat oven to 350° F. Spread the butter on one side of each slice of bread, then lay 4 of them on a greased baking sheet, butter-side down.
Divide the chicken among the 4 slices and top with some of the cheese and apple slices. Place the remaining bread slices on top, butter-side up. Bake for 10 minutes. Turn and bake until crispy and golden, about 10 minutes more. Cut in half and serve immediately.