Preheat oven to 350°F.
Cook butter and garlic in a medium skillet over medium-low heat, swirling pan often, until foaming subsides and garlic is fragrant, about 2 minutes. Add panko and stir to coat evenly. Cook, stirring often, until panko is golden brown, about 4 minutes. Discard the garlic. Transfer bread crumbs to a plate to cool while you prepare the rest of the ingredients.
In a large bowl, combine cooked pasta, shredded chicken, marinara sauce, salt, pepper, 1 cup of mozzarella and ½ cup Parmesan.
Transfer mixture to a greased 12-inch cast iron skillet or large baking dish. Cover with foil and bake until heated through and bubbly, about 25 minutes.
Remove the cover and sprinkle with remaining 1 cup of mozzarella, remaining ¼ cup of Parmesan, and toasted bread crumbs. Return to the oven until the cheese melts, about 5 more minutes.
Let stand for about 5-10 minutes. Garnish with chopped fresh basil or parsley just before serving.