Part 1: The Pasta – The Undercooking Rule

This is the most critical step for make-ahead success.

1. Boil Water: Bring a large pot of heavily salted water to a rolling boil.

2. Undercook the Pasta: Add the pasta and cook according to package directions, but pull it from the water 2 to 3 minutes before it reaches al dente. It should still be quite firm and have a significant bite.

3. Drain, Don’t Rinse: Drain the pasta well, but do not rinse it. The starches on the surface will help the cheese sauce cling to it. Set aside.

Part 2: The Velvety Cheese Sauce

1. Grate the Cheese: While the pasta is cooking, grate your cheeses and toss them together in a large bowl. Do not use pre-shredded cheese. The anti-caking agents in pre-shredded cheese can make your sauce grainy.

2. Make the Roux: In a large Dutch oven or heavy-bottomed pot, melt the 6 tablespoons of butter over medium heat. Once melted, whisk in the ⅓ cup of flour. Cook, whisking constantly, for 1-2 minutes. This cooks out the raw flour taste. The mixture (roux) should be smooth and bubbly.

3. Create the Béchamel: Gently warm the milk in a separate saucepan or in the microwave. Gradually pour the warm milk into the roux, whisking vigorously and constantly to prevent lumps. Continue pouring and whisking until all the milk is incorporated and the sauce is smooth.

4. Thicken the Sauce: Continue cooking the sauce over medium heat, stirring frequently, for about 5-8 minutes, until it has thickened enough to coat the back of a spoon.

5. Add the Cheese: Remove the pot from the heat. This is a crucial step to prevent the cheese from separating and becoming oily. Add the grated cheese to the sauce in handfuls, stirring with a wooden spoon or spatula until each addition is fully melted and incorporated before adding the next.

6. Season to Perfection: Once all the cheese is melted and the sauce is velvety smooth, stir in the dry mustard, smoked paprika, salt, black pepper, and optional cayenne pepper. Taste and adjust seasoning if necessary.

Part 3: The Assembly (The Make-Ahead Step)

1. Combine and Conquer: Pour the undercooked pasta into the pot with the cheese sauce. Stir gently until every piece of pasta is thoroughly coated. At this point, the mixture should look extremely saucy, almost like a soup. This is intentional! The pasta will absorb a significant amount of sauce as it cools and rests in the fridge.

2. Prepare for Chilling: Transfer the mac and cheese mixture to a 9×13-inch baking dish. Spread it into an even layer.

3. Cool Completely: Allow the mac and cheese to cool completely at room temperature, for at least 1 hour. Placing a hot dish directly into the fridge can create condensation, which can make the top watery.

4. Store for Later: Once completely cool, cover the dish tightly with foil or plastic wrap and refrigerate for up to 2 days.

Part 4: The Final Bake (Day of Serving)

1. Preheat and Prep: Remove the mac and cheese from the refrigerator and let it sit on the counter for 30-60 minutes to take the chill off. This helps it bake more evenly. Preheat your oven to 375°F (190°C).

2. Make the Topping: In a small bowl, combine the Panko breadcrumbs, melted butter, grated Parmesan cheese, and optional fresh parsley. Toss until the breadcrumbs are evenly moistened.

3. Top and Bake: Sprinkle the breadcrumb mixture evenly over the surface of the mac and cheese.

4. Bake to Bubbly Perfection: Cover the dish with foil and bake for 20-30 minutes. Then, remove the foil and continue to bake for another 15-20 minutes, or until the sauce is hot and bubbly and the topping is a deep golden brown and crispy.

5. Rest Before Serving: Let the mac and cheese rest for 10-15 minutes before serving. This allows the sauce to set up slightly, ensuring a creamy, not runny, consistency.