01 - Spread softened butter evenly onto one side of each bread slice.
02 - Place two bread slices, butter-side down, on a clean surface. Layer with mozzarella, tomato slices, and basil leaves. Season tomatoes and cheese with salt and black pepper.
03 - Drizzle one tablespoon of balsamic glaze over the tomato layer on each sandwich.
04 - Sprinkle with grated Parmesan if desired, then cover with the remaining bread slices, butter-side up.
05 - Heat a nonstick skillet over medium heat. Place assembled sandwiches in the skillet and grill for 3 to 4 minutes per side, pressing gently with a spatula, until bread is golden and cheese is melted.
06 - Remove sandwiches from heat, let rest for one minute, slice, and serve warm.