Heat a your pan/skillet over medium heat. (Non-stick is a must!)
Butter one side of each bread slice, then spread the pesto on the other side. Sandwich a slice of mozzarella between the basil sides (so the butter sides of the bread are facing out.) Sprinkle half of the Parmesan on the outside of one side of the sandwich.
Place the sandwich, cheese side down, on the hot pan. Grill for 2 minutes or until Parmesan is crusted and golden brown. Flip and sprinkle the rest of the Parmesan onto the other side of the sandwich.Cook about 2 more minutes and serve it up!