Preheat oven to 400 degrees F (205 degrees C). Butter a 1 1/2 quart casserole dish. Bring a saucepan of lightly salted water to a boil. Add macaroni, and cook until not quite done, about 6 minutes. Drain.
In a separate saucepan, melt the butter over medium heat. Blend in the flour, mustard powder, and pepper until smooth. Slowly stir in the milk, beating out any lumps. Add the American and processed cheeses, and stir constantly until the sauce is thick and smooth.
Drain noodles, and stir them into the cheese sauce. Transfer the mixture to the prepared casserole dish. Sprinkle bread crumbs over the top.
Cover the dish, and bake for 20 to 25 minutes, or until sauce is thick and bubbly.
