01 - Spread 1 tablespoon of softened butter evenly on one side of each bread slice, ensuring complete coverage to the edges.
02 - Flip each bread slice and spread approximately 1½ tablespoons of basil pesto on the unbuttered side of two slices.
03 - Layer 2 slices of mozzarella cheese over the pesto, then sprinkle 1 tablespoon of grated Parmesan on top. Add tomato slices or fresh spinach if desired.
04 - Top each assembled sandwich with the remaining bread slices, placing the buttered side facing outward for optimal grilling.
05 - Heat a non-stick skillet over medium heat. Place sandwiches in the skillet and cook for 3-4 minutes per side, pressing gently with a spatula until bread achieves golden brown color and cheese melts completely.
06 - Remove sandwiches from skillet and let rest for 1 minute to set. Slice diagonally and serve warm.