Preheat oven to 350 degrees. Grease a 9x13 casserole dish with unsalted butter and set aside.
Boil macaroni according to directions on package.
Remove the macaroni from heat earlier than fully cooked (al dente or slightly more firm than that). Drain; do not rinse.
While the macaroni is cooking, melt ½ cup butter on medium low heat. Once butter is melted and slightly bubbly, stir in flour until the mixture is smooth and bubbly.
Slowly add milk, whisking vigorously the entire time.
Turn up the heat to medium/medium high. Boil for one minute, stirring. This should form a
Stir in Worcestershire sauce, mustard, salt, and pepper. Remove from heat.
Add cheese to the sauce, a little at a time, blending until the mixture is smooth and creamy.
Add pasta and turn to coat.
Pour pasta into a greased casserole dish. Mix 3 tablespoons melted butter with bread crumbs and sprinkle on top.
Bake in oven for 30-35 minutes until bubbly. Turn the heat on the oven to a hi broil and set an oven timer for 2 minutes. Broil the top of the panko until it's slightly browned; you'll have to keep a careful eye on it to make sure that the panko doesn't scorch (it took my oven about 3-4 minutes, but I still set an oven timer in case I got distracted). Garnish with parsley (if using) and serve immediately.
Recipe by Grits and Chopsticks at https://www.gritsandchopsticks.com/the-gravy-train/2015/11/classic-mac-and-cheese.html