Bring a large pot of salted water to a boil. Add the macaroni and cook according to package directions until al dente, about 7–8 minutes. Drain and set aside.

While the pasta cooks, melt butter in a medium saucepan over medium heat. Once melted, whisk in the flour and cook, whisking constantly, for 1 minute until bubbling but not browned.

Gradually add the milk to the flour-butter mixture (roux), whisking to remove any lumps. Continue to cook, whisking, until the mixture thickens, about 4–5 minutes.

Remove the saucepan from heat. Stir in the salt, black pepper, and ground mustard (if using). Add all the cheddar and half of the mozzarella, stirring until completely melted and smooth.

Add the drained macaroni to the cheese sauce, stirring gently until the pasta is evenly coated.

For extra cheesiness and a crispy top (optional): Transfer mac and cheese to a baking dish, sprinkle with the remaining mozzarella, and broil in the oven for 2–3 minutes until golden and bubbly.

Serve hot and enjoy!