Step 1: Cook the Pasta and Prepare the Chicken
Preheat your oven to 375°F (190°C). Butter a 9×13 inch (23x33cm) baking dish or a 5-quart Dutch oven. Bring a large pot of heavily salted water to a boil. Cook the pasta for 2-3 minutes less than the package directions for al dente. It will finish cooking in the sauce. Drain and set aside. In a bowl, toss the shredded chicken with olive oil, smoked paprika, garlic powder, salt, and pepper. Set aside.

Step 2: Create the Roux – The Foundation of the Sauce
In a large, heavy-bottomed pot or Dutch oven, melt the ½ cup of butter over medium heat. Once foamy, whisk in the flour. Cook, whisking constantly, for 3-5 minutes until the mixture (the roux) turns a light golden blonde and smells nutty. This cooks out the raw flour taste and is crucial for flavor.

Step 3: Build the Creamy Béchamel
Gradually pour in the warm milk and half-and-half, whisking constantly to ensure no lumps form. Whisk in the dry mustard, onion powder, nutmeg, and optional cayenne. Continue to cook, whisking frequently, until the sauce thickens enough to coat the back of a spoon, about 8-10 minutes. It should be smooth and velvety. Remove from heat. Season with the 2 teaspoons of kosher salt and pepper.

Step 4: Create the Velvety Cheese Sauce
Take the pot off the heat. It is crucial to do this before adding cheese to prevent separation. Gradually add the shredded cheddar, Monterey Jack, and Gruyère cheeses, along with ¾ cup of the Parmesan cheese, one handful at a time, whisking until each addition is fully melted and smooth before adding the next. The residual heat will melt the cheese into a perfectly smooth, glossy Mornay sauce.

Step 5: Integrate the BBQ Flavor and Assemble
Stir 1 ½ cups of the BBQ sauce into the cheese sauce until fully incorporated. Taste and add more BBQ sauce if desired, remembering the flavor will concentrate slightly during baking. Add the drained al dente pasta and seasoned chicken to the cheese-BBQ sauce. Gently fold everything together until every piece is thoroughly coated.

Step 6: Prepare the Crunchy Topping and Final Assembly
Pour the creamy BBQ chicken mac and cheese mixture into the prepared baking dish, spreading it evenly. In a small bowl, combine the Panko breadcrumbs, melted butter, and the remaining ¼ cup of grated Parmesan cheese. Sprinkle this mixture evenly over the entire surface of the casserole. For an extra touch, drizzle the remaining ¼ to ½ cup of BBQ sauce in artistic lines over the breadcrumb topping.

Step 7: Bake to Golden-Brown, Bubbling Perfection
Place the baking dish on a rimmed baking sheet (to catch any potential bubble-overs). Bake in the preheated oven for 25-30 minutes, or until the topping is deeply golden brown, the edges are bubbling vigorously, and the center is hot. If the top browns too quickly, tent it loosely with foil.

Step 8: Rest, Garnish, and Serve
Remove from the oven and let the casserole rest for 10-15 minutes before serving. This allows the sauce to thicken slightly and set, ensuring clean, beautiful servings. Garnish generously with sliced green onions and fresh parsley or chives just before serving