Preheat your oven to 350°F (175°C).
Cook the macaroni according to package instructions until al dente, then drain.
In a large saucepan, melt butter over medium heat. Add flour, whisking constantly for about 2 minutes.
Slowly pour in the milk while continuing to whisk, and cook until the mixture thickens.
Stir in the cheddar cheese, garlic powder, Dijon mustard, salt, and pepper until smooth and melted.
Combine the cooked macaroni with the cheese sauce, mixing well to coat all pasta evenly.
Transfer the mixture to a greased 9x13 inch baking dish.
In a small bowl, mix the breadcrumbs with Parmesan cheese, then sprinkle over the macaroni mixture.
Bake in the preheated oven for 25-30 minutes, or until the topping is golden brown and the cheese is bubbling.
Let it rest for about 10 minutes before serving.