Heat oven to 350º F.
Spread the butter on one side of each slice of bread. Lay 4 slices on an ungreased 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.