01 - Spread 1 tablespoon of softened butter evenly on one side of each bread slice.
02 - Flip the bread slices and spread about 1½ tablespoons of basil pesto on the unbuttered side of two slices.
03 - Layer 2 slices of mozzarella cheese on each pesto-covered slice and sprinkle 1 tablespoon of grated Parmesan on top. Add tomato slices or fresh spinach if desired.
04 - Top each sandwich with the remaining bread slices, buttered side facing outward.
05 - Heat a non-stick skillet over medium heat. Place the sandwiches in the pan and cook for 3 to 4 minutes per side, pressing gently with a spatula, until the bread is golden brown and the cheese is fully melted.
06 - Remove from the skillet and let rest for 1 minute before slicing. Serve warm.