01 - Spread one tablespoon of pesto evenly on the inside surface of each bread slice.
02 - Top two pesto-coated bread slices with an even layer of mozzarella slices and sun-dried tomatoes if using. Place the remaining bread slices on top, pesto side down.
03 - Spread one tablespoon of butter evenly over the outside of each sandwich.
04 - Heat skillet over medium heat. Place sandwiches in skillet and cook 2–3 minutes per side, pressing gently, until bread is golden brown and cheese is fully melted.