Preheat oven to 375°F (190°C) and butter a 9 by 13 inch baking dish. Cook pasta in salted boiling water 2 minutes less than package directions, drain, and rinse with cold water.

Melt butter in a saucepan, whisk in flour and cook 1 minute. Slowly whisk in cold milk and broth, bring to a simmer, and cook 4 to 5 minutes until thickened. Off heat, stir in cheddar, Parmesan, mustard, garlic powder, salt, and pepper until smooth.

Fold the pasta and shredded chicken into the sauce, then transfer to the prepared dish.

Toss panko with olive oil, scatter over the top, and bake 20 to 25 minutes until bubbling and golden.

Let the casserole rest 5 minutes, then scoop and serve hot.