Mix tapenade with orange zest and chile flakes in a small bowl.
Layer mozzarella with tapenade on 2 bread slices. Top with remaining slices.
Butter outsides of each sandwich.
Heat a large well-seasoned cast-iron skillet or a large, heavy nonstick pan over medium heat for a minute.
Place sandwiches in skillet and cook until golden-brown underneath, 5 to 7 minutes. Flip and cook the other side until it's just as toasty. Low and slow is the key. Cheese will probably ooze out, but that's okay. It gives the sandwiches a nice crunchy fringe.