Preheat your oven to 375°F. Lightly grease a casserole dish or a 9x13-inch baking pan to prevent sticking.

Cook the macaroni in a large pot of salted boiling water according to the package directions until tender. Drain thoroughly and set aside.

While the pasta is cooking, mix 2½ cups of the shredded cheese with the flour, salt, ground black pepper, and ground mustard in a medium bowl. Toss to evenly coat the cheese in the dry ingredients.

In a large mixing bowl, combine the cooked macaroni with the cheese and flour mixture. Stir until the cheese begins to melt slightly and coats the pasta evenly. Transfer the mixture into the prepared casserole dish, spreading it out evenly.

Pour the milk evenly over the macaroni. Sprinkle the remaining cheese over the top, ensuring it covers the surface. Dot small pieces of butter across the top for added richness. Cover the dish with aluminum foil to lock in moisture.

Bake in the preheated oven for 45 minutes, removing the foil after 30 minutes to allow the top to become golden brown and slightly crisp. Check the center with a knife to ensure the milk has been mostly absorbed—only a minimal amount should remain as the dish will continue to set while cooling.

(Optional) Let the baked macaroni and cheese rest for 10 minutes before serving. This allows the dish to firm up for cleaner servings. Enjoy warm for a creamy and comforting meal!