Boil the elbow macaroni according to package directions. Add the broccoli florets during the last 3 minutes of cooking. Before draining, reserve 1/2 cup of pasta water. Drain pasta and broccoli, and set aside.
In a large pot, melt the butter over medium heat. Add the flour, whisking constantly for about 1-2 minutes to form a roux. Slowly whisk in the milk and cook, stirring constantly, until the mixture thickens (about 3-5 minutes).
Reduce the heat to low, or turn it off, and gradually add the shredded cheddar and mozzarella cheeses, a handful at a time. Stir constantly, letting each handful melt before adding more to keep the sauce smooth.
Stir in the garlic powder, onion powder, mustard powder, salt, and pepper. If the sauce is too thick, add a little of the reserved pasta water until you reach your preferred consistency.
Add the cooked macaroni and broccoli to the cheese sauce, stirring until everything is well-coated.
Optional - Sprinkle cheddar cheese on top before serving for extra flavor.
