Cook the Chicken: Season the chicken breasts with salt and pepper on both sides. Heat olive oil in a skillet over medium heat. Add the chicken breasts and cook for 5-6 minutes per side, or until the internal temperature reaches 165°F (75°C) and the chicken is fully cooked. Once cooked, remove the chicken from the skillet, slice it thinly, and set it aside.
Assemble the Grilled Cheese: Spread butter on one side of each slice of bread. Place one slice of bread, buttered side down, on a flat surface. Layer with shredded mozzarella, followed by sliced chicken, marinara sauce, and a sprinkle of Parmesan cheese. Top with a few basil leaves if desired. Place the second slice of bread on top, buttered side up.
Grill the Sandwiches: Heat a skillet or griddle over medium-low heat. Grill each sandwich for 3-4 minutes per side, pressing down lightly with a spatula to ensure even browning and melting. Grill until the bread is golden brown and crispy, and the cheese is melted.
Serve and Enjoy: Slice the sandwiches in half and serve immediately with a side of marinara sauce for dipping. Garnish with fresh basil, if desired.