Preheat the oven to 350°F (175°C).
In a large pot, bring salted water to a boil and cook the elbow macaroni according to the package instructions until al dente, usually about 6-8 minutes. Drain the pasta in a colander and set aside.
In a saucepan, melt 4 tablespoons of unsalted butter over medium heat. Once melted, add the flour and whisk continuously for about 1 minute to create a roux.
Gradually add the milk, whisking constantly to prevent lumps. Continue cooking until the sauce thickens, about 5 minutes.
Remove the sauce from heat and stir in the shredded sharp cheddar and mozzarella cheese until melted and smooth. Season with garlic powder, onion powder, mustard powder, salt, and pepper.
Add the cooked macaroni to the cheese sauce, stirring until well combined.
Transfer the mixture to a greased 9x13-inch baking dish.
In a small bowl, combine the breadcrumbs with 2 tablespoons of melted butter. Mix well, then sprinkle over the mac and cheese.
Bake in the preheated oven for 20-25 minutes, or until the top is golden brown and the cheese is bubbling.
Allow to cool for a few minutes before serving.