Cook the pasta: Bring a pot of salted water to a boil. Add the pasta and cook until al dente; then drain and set aside.
Make the roux: Meanwhile, melt butter in a saucepan over medium heat. Whisk in flour and cook for 1–2 minutes until smooth and slightly golden.
Add milk: Next, gradually pour in the milk while whisking continuously until the sauce thickens and becomes creamy.
Add cheese: Then, lower the heat and stir in the cheddar and mozzarella until melted. Season with salt, pepper, and mustard powder.
Combine: Finally, fold in the cooked pasta until evenly coated. Serve immediately, or top with breadcrumbs and bake for 10 minutes for a golden crust. Tips & Variations Add cooked bacon, broccoli, or peas for extra flavor and texture.
Use a mix of cheeses like gouda, parmesan, or Monterey Jack for a richer sauce.
For a crispier topping, broil for a few minutes before serving.