01 - Spread the softened butter on one side of each bread slice. Arrange the slices, buttered side down, on a clean surface.
02 - On the unbuttered side of two bread slices, spread 1 tablespoon of basil pesto evenly.
03 - Place fresh mozzarella slices, followed by tomato slices and basil leaves on top of the pesto-coated bread. Season lightly with sea salt and black pepper.
04 - Cover with the remaining bread slices, ensuring the buttered surface is facing outward.
05 - Preheat a large nonstick skillet or grill pan over medium heat.
06 - Place assembled sandwiches onto the skillet. Cook for 3 to 4 minutes per side, gently pressing with a spatula, until bread is golden and cheese fully melts.
07 - Remove sandwiches from the pan and let them rest for 1 minute. Slice in half and serve immediately.