Step 1: Cook the pasta. Bring a large pot of salted water to a rolling boil. Add 1 pound of elbow macaroni and cook according to package directions until al dente. Drain well and set aside, do not rinse.
Step 2: Prepare the chicken. In a medium bowl, toss the cooked, shredded chicken with 1 cup of your favorite BBQ sauce until well coated. Set aside.
Step 3: Make the roux. In a large, deep pot or Dutch oven, melt 1/2 cup of unsalted butter over medium heat. Gradually whisk in 1/2 cup of all-purpose flour. Cook, stirring constantly, for 1-2 minutes until a thick paste forms.
Step 4: Add milk. Slowly whisk in 4 cups of whole milk, a little at a time, ensuring no lumps form. Continue whisking until the sauce is smooth and begins to thicken, about 5-7 minutes. Bring it to a gentle simmer, but do not boil.
Step 5: Stir in cheese and seasonings. Reduce the heat to low. Add 4 cups of grated cheese blend (sharp cheddar, smoked Gouda, Monterey Jack) to the sauce, a handful at a time, whisking until fully melted and incorporated. Season with 1 teaspoon of salt, 1/2 teaspoon of black pepper, 1/4 teaspoon of garlic powder, and an optional pinch of cayenne pepper. Taste and adjust seasonings.
Step 6: Combine pasta and sauce. Add the drained cooked pasta to the cheese sauce and stir gently until every noodle is thoroughly coated.
Step 7: Layer for baking. Pour half of the mac and cheese mixture into a greased 9x13 inch baking dish. Evenly spread the BBQ chicken mixture over this layer. Top with the remaining mac and cheese.
Step 8: Add topping (optional). If desired, sprinkle an additional 1 cup of grated cheese over the top, along with 1/2 cup of panko breadcrumbs mixed with 1 tablespoon of melted butter for a crispy crust.
Step 9: Bake. Preheat your oven to 375°F (190°C). Bake for 25-30 minutes, or until the top is golden brown and bubbly. If browning too quickly, tent loosely with foil.
Step 10: Rest and serve. Remove from the oven and let rest for 5-10 minutes before serving. Garnish with fresh chopped chives or parsley, if desired.