Preheat the oven to 400°F (204°C).
Bring a pot of salted water to a boil, cook the macaroni until al dente according to package instructions, 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 constantly, for about 1 minute to make a roux.
Slowly whisk in the milk and continue to cook, stirring, until the sauce thickens and is smooth.
Add the American cheese and stir until it melts into the sauce.
Stir in the salt, black pepper, and dry mustard, then add the cooked macaroni and toss until evenly coated with the cheese sauce.
Lightly butter a 2-quart casserole dish, pour in the macaroni mixture, and spread evenly.
Bake in the preheated 400°F oven for 20 minutes until heated through and slightly bubbly.
