Coat one side of each slice of bread evenly with softened butter. Lightly dust the buttered surfaces with grated Parmesan to enhance flavor and crispness.
Lay two slices of bread, buttered side down, on a flat surface. Spread a thin layer of marinara or pizza sauce across the top-facing, unbuttered side.
Add mozzarella cheese atop the sauce, followed by slices of pepperoni and a light sprinkle of Italian seasoning, if using.
Close the sandwiches with the remaining bread slices, keeping the buttered sides facing outward.
Warm a nonstick skillet or griddle over medium heat. Place the assembled sandwiches on the hot surface and toast for 3 to 4 minutes on each side, or until golden brown and the cheese is thoroughly melted.
Remove from heat, slice each sandwich in half, and serve immediately. Accompany with additional warm marinara sauce for dipping, if desired.