Preheat the oven to 400°F (200°C).
Bring a pot of salted water to a boil and cook the macaroni until al dente according to package directions; drain and set aside.
In a large saucepan over medium heat, melt the butter until fragrant.
Add the flour to the melted butter and cook, stirring, for about 1 minute to form a roux.
Slowly whisk in the milk and continue cooking, stirring, until the sauce thickens.
Add the American cheese and stir until it melts into a smooth sauce.
Stir in the salt, black pepper, and dry mustard, then add the drained macaroni and toss until evenly coated with the cheese sauce.
Lightly butter a 2‑quart casserole dish, transfer the macaroni mixture to the dish, and smooth the top.
Bake in the preheated oven for 20 minutes, until bubbly and heated through.