Preheat the oven to 350° F.
Cook the macaroni using package instructions until still slightly firm. Drain and set aside.
In a small bowl, beat egg. In a large pot, melt the butter and sprinkle in flour. Whisk together over medium-low heat. Cook for a couple of minutes, whisking constantly. Don’t let it burn. Pour in the milk, add the mustard and whisk until smooth. Cook until very thick, about 5 minutes. Reduce the heat to low.
Take ¼ cup of the sauce and slowly pour it into the beaten egg, whisking constantly. Stir until smooth. Add in the cheese and stir to melt. Add ½ teaspoon salt, ½ teaspoon seasoned salt and the pepper. Taste the sauce and add more salt and seasoned salt as needed. DO NOT UNDERSALT!
Pour in the drained, cooked macaroni and stir to combine.
Pour macaroni mixture into a buttered baking dish. Sprinkle on bread crumb topping mixture. Cover with aluminum foil. Bake 30 minutes covered. Remove foil. Bake for another 30-45 minutes until topping is brown and Mac ‘n Cheese is bubbly.