Preheat oven to 350°F and grease a 9x13 baking dish.
Cook thin spaghetti 1 minute less than package directions. Drain and rinse with cold water.
In a large bowl, combine melted butter, chicken, cream of chicken soup, sour cream, milk, salt, pepper, and Parmesan.
Fold in drained spaghetti until evenly coated.
Spread mixture in the prepared baking dish and top with mozzarella.
Bake 30 minutes, until edges are bubbling and cheese is melted.
Sprinkle Panko over the top and bake 10 more minutes, until golden and crisp.
Let rest 5 minutes before serving.
Notes
Cook the pasta slightly underdone because it finishes in the oven. Rinse the pasta after draining to stop cooking and prevent clumping. Add Panko only during the last 10 minutes so it stays crisp. Let the casserole rest before serving for cleaner scoops.