01 - Arrange the bread slices on a clean surface and spread 1 tablespoon of basil pesto onto two of the slices.
02 - Top the pesto-covered slices evenly with mozzarella cheese.
03 - Cover the cheese with the remaining slices of bread to assemble the sandwiches.
04 - Spread softened butter evenly on the outside of each sandwich on both sides.
05 - Heat a large nonstick skillet or grill pan over medium heat.
06 - Place the sandwiches in the heated pan. Cook for 3 to 4 minutes on each side, pressing gently, until the bread is golden brown and the cheese is melted.
07 - Transfer grilled sandwiches to a cutting board. Let rest for 1 to 2 minutes, slice as desired, and serve promptly while warm.