Boil the spaghetti according to the package directions until al dente. Reserve 1 cup of the pasta water and then drain the spaghetti. Do not rinse, set aside.
In a small bowl, combine the melted butter, panko breadcrumbs, remaining Parmesan cheese and the parsley and stir together. Sprinkle the mixture evenly over top of the pasta. Transfer the casserole dish to the oven.
Let the casserole sit for at least 10 minutes before serving. Garnish with more chopped parsley if desired. Serve hot.