Preheat the oven to 400°F (200°C).
Bring a large pot of salted water to a boil, add the macaroni, and cook until al dente according to package directions; drain and set aside.
In a large saucepan over medium heat, melt the butter until fragrant but not browned.
Whisk in the flour and cook about 1 minute to form a roux.
Gradually whisk in the milk and cook, stirring frequently, until the sauce thickens and is smooth.
Add the American cheese and stir until fully melted into the sauce.
Stir in the salt, black pepper, and dry mustard, then add the drained pasta and toss until evenly coated with the cheese sauce.
Lightly butter a 2-quart casserole dish, transfer the pasta mixture to the dish, and spread evenly.
Bake in the preheated oven for 20 minutes, until bubbly and lightly golden at the edges; remove and serve.