01 - Preheat the oven to 350°F. Lightly grease an 8x8 inch baking dish.
02 - Boil salted water and cook elbow macaroni according to package instructions until al dente. Drain and set aside.
03 - Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 minute, ensuring it bubbles without browning.
04 - Gradually whisk in milk, stirring constantly, and cook until the sauce thickens, about 3 to 5 minutes.
05 - Remove from heat and stir in cheddar, optional Gruyère, salt, pepper, and optional mustard until fully melted and smooth.
06 - Add the drained macaroni to the cheese sauce, stirring until the pasta is evenly coated.
07 - Transfer the macaroni mixture to the prepared baking dish.
08 - In a small bowl, mix breadcrumbs, melted butter, and optional Parmesan cheese; sprinkle evenly over the macaroni.
09 - Bake for 20 minutes or until the top is golden and bubbling. Allow to rest for 5 minutes before serving.