Preheat the oven to 375°F (190°C) and lightly grease a 9-inch square or 2-quart baking dish.
In a small bowl, combine the melted butter with panko breadcrumbs, garlic powder, Parmesan cheese, and parsley to form the topping. Set this mixture aside.
Boil the pasta until just al dente, following the instructions on the package. Drain well.
In a large mixing bowl, combine the cooked pasta with 2 cups of the marinara sauce, the Italian seasoning blend, pre-cooked chicken, drained diced tomatoes (if using), and a tablespoon of chopped parsley. Stir to evenly coat all ingredients.
Transfer the pasta mixture into the prepared baking dish. Spoon the remaining sauce over the top surface.
Sprinkle the mozzarella cheese over the pasta, followed by the breadcrumb topping mixture.
Bake uncovered for 23–25 minutes, or until the topping turns a rich golden brown and the casserole is bubbling at the edges.