Preheat the oven to 425°F.
Cook the macaroni al dente according to the package instructions, then drain and rinse under cold water.
Melt the butter in a large saucepan over medium heat, whisk in the flour, and cook for two minutes while stirring. Gradually whisk in the milk, cream, mustard powder, onion powder, salt, and pepper, then continue cooking and stirring until the mixture thickens.
Remove the pan from the heat and stir in the Parmesan and 3 cups of cheddar cheese until melted. Incorporate the soup if using, then taste and adjust the seasoning with salt and pepper as needed.
Combine the cheese sauce and macaroni noodles, transfer the mixture to a greased 9x13-inch baking pan, and sprinkle the remaining cup of cheese on top.
Bake for 18 to 24 minutes or until bubbly, taking care not to overcook, and let the dish cool for 10 to 15 minutes before serving.
