Butter one side of each slice of bread evenly.
Place one slice of bread, buttered side down, on a preheated grill or skillet.
Layer the mozzarella cheese slices on top of the bread.
Cover with the second slice of bread, buttered side up.
Grill over medium heat for about 4 minutes, until the bottom is golden brown and crispy.
Carefully flip the sandwich and grill the other side for another 4 minutes until golden and the cheese is melted and gooey.
Remove from heat and let rest for a minute before cutting diagonally into halves.
Stack the halves slightly offset on a plain white plate for serving.