1. Slice tomatoes and mozzarella evenly and pat dry on paper towels to remove excess moisture.

2. Rinse and pat dry basil leaves, removing any thick stems.

3. Preheat a cast-iron skillet or grill pan over medium heat until a drop of butter sizzles.

4. Spread butter or brush olive oil evenly on one side of each bread slice.

5. Place two slices of bread, buttered side down, on a cutting board.

6. Layer mozzarella on each slice, followed by tomato slices, basil leaves, salt, and black pepper.

7. Drizzle balsamic glaze or spread pesto if using, then top with remaining bread slices, buttered side up.

8. Place sandwiches in the hot skillet and cook for 3-4 minutes per side, pressing gently, until bread is golden and cheese is melted.

9. If bread browns too quickly, lower heat and cover pan for the last minute to melt cheese.

10. Transfer sandwiches to a board and let rest for 1 minute before slicing diagonally and serving.