Preheat oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter or cooking spray.
Cook Mueller's Elbows according to package directions until al dente. Drain and set aside.
In a medium saucepan, melt butter over medium-high heat. Whisk in flour, salt (optional), dry mustard, and pepper until combined; cook for 1 minute, stirring constantly. Gradually whisk in milk and bring to a boil, whisking constantly. Boil for 1 minute or until the mixture is thickened.
Remove from heat and stir in shredded cheese until melted. Add cooked macaroni and mix to combine. Pour into prepared baking dish and spread evenly.
Bake for 20 minutes or until the top is golden brown and bubbling. Let cool for 5 minutes before serving.