Preheat the oven to 350 degrees F. Butter a 9-by-13-inch baking dish.

In a large pot over high heat, bring about 2 quarts of water to a boil and sprinkle in the kosher salt. Add the macaroni and cook until it is al dente (cooked but still firm). Drain the pasta and transfer it to a large mixing bowl.

While the pasta is cooking, melt 4 tablespoons of the butter in a large saucepan over medium heat. Sprinkle in the flour and whisk until there are no lumps. Pour in the half-and-half and whisk until smooth. Cook until the mixture starts to thicken, then reduce the heat to medium-low.

Add 8 ounces of the cheddar cheese and whisk until well incorporated. Sprinkle in the seasoning salt and black pepper and stir. Pour the cheese sauce over the cooked macaroni and fold it in until well incorporated.

In a small bowl, toss the remaining 8 ounces cheddar with the mozzarella. Sprinkle a layer of this mozzarella mixture on the bottom of the prepared baking dish, then top with a heavy layer of macaroni and cheese. Repeat, finishing with a final layer of the mozzarella mixture.

In a small saucepan or microwaveable bowl, melt the remaining 2 tablespoons butter. Add the bread crumbs and stir. Sprinkle the mixture over the casserole.

Bake, uncovered, for 25 to 30 minutes, or until the bread crumbs are golden brown and crisp. Serve and enjoy the best-ever Southern baked macaroni and cheese casserole!!