Cook macaroni according to package direction for al dente. Reserve ½ cup of the cooking water from the pasta – drain and rinse.
Preheat oven to 350F.
In the same kettle used to cook the pasta mix together the milk, mustard, garlic powder, and cayenne pepper. Dissolve the cornstarch into a ¼ cup of the pasta water until dissolved.
Bring the milk mixture to a simmer and whisk in the cornstarch. Cook until mixture starts to thicken. Remove the mixture from the heat and stir in the cheese and macaroni. Stir in the rest of the pasta water to make the macaroni mixture kind of soupy. (I did not need the rest of the water)
Place the macaroni and cheese in a 13×9 casserole dish. Toss the toasted Panko bread crumbs with the melted butter, salt, and pepper to taste, and spread them over the top of the casserole.
Bake for 20 minutes or until mixture is bubbly around the edges.