1. Prepare the Pasta: Begin by cooking the elbow macaroni according to package directions until al dente. Drain the pasta thoroughly and set it aside. Do not rinse the pasta; the starch helps the sauce cling better.

2. Create the Roux: In a large, oven-safe pot or Dutch oven, melt 1/2 cup of unsalted butter over medium heat. Once melted, sprinkle in the 1/2 cup of all-purpose flour. Whisk continuously for 1-2 minutes until a smooth paste, or roux, forms. This step is crucial for thickening your cheese sauce.

3. Whisk in the Milk and Seasonings: Gradually whisk in the 4 cups of warmed whole milk, a little at a time, ensuring no lumps form. Continue whisking until the sauce begins to thicken, about 5-7 minutes. It should be thick enough to coat the back of a spoon. Stir in the salt, black pepper, ground mustard, and paprika.

4. Melt the Cheeses: Reduce the heat to low. Add the 4 cups of shredded sharp cheddar cheese and 2 cups of shredded Monterey Jack cheese to the sauce. Stir gently until all the cheese is completely melted and the sauce is smooth and creamy. Taste and adjust seasonings if necessary.

5. Combine Pasta and Sauce: Add the cooked and drained macaroni to the cheese sauce. Stir well to ensure every piece of pasta is thoroughly coated in the rich, cheesy goodness.

6. Prepare for Baking: If your pot is oven-safe, you can proceed directly. Otherwise, transfer the mac and cheese mixture to a 9×13 inch baking dish. In a small bowl, combine the 1 cup of panko breadcrumbs with 2 tablespoons of melted butter. Sprinkle this mixture evenly over the top of the mac and cheese.

7. Bake to Golden Perfection: Bake in a preheated oven at 375 F (190 C) for 20-25 minutes, or until the top is golden brown and bubbly. The breadcrumb topping should be crispy, and the cheese sauce should be hot throughout.

8. Rest and Serve: Once baked, remove the mac and cheese from the oven and let it rest for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve and preventing it from being too runny.