Cook the macaroni according to package instructions. Drain and set aside.
In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes until lightly golden, stirring constantly.
Gradually whisk in the milk, making sure there are no lumps. Continue to cook, stirring constantly, until the mixture thickens and begins to bubble.
Reduce the heat to low and add the white cheddar cheese and Parmesan cheese. Stir until the cheeses are completely melted and the sauce is smooth.
Stir in the Dijon mustard and garlic powder, and season with salt and pepper to taste.
Add the cooked macaroni to the cheese sauce and stir until well combined.
If desired, transfer the mac and cheese to a baking dish, sprinkle with breadcrumbs, and bake at 350°F for 15-20 minutes until the top is golden and crispy.
Garnish with chopped parsley before serving, if desired.