Heat the oven to 350°F, then stir the 2 tbsp butter (melted), 1 cup Panko bread crumbs, 1/2 cup bleu cheese crumbles, and 1/4 cup parsley chopped (loosely packed) together in a small bowl. The topping should look evenly moistened with loose crumbs, not pasty or packed.

Boil the 8 oz elbow macaroni in heavily salted water following the package directions, then drain and toss it with 1 tbsp olive oil. Cook the pasta to al dente so it stays tender in the oven instead of turning soft.

Melt 4 tbsp butter in a large skillet over medium heat, then cook the onion and celery for 5 minutes before adding the garlic, 1/2 tsp salt, and 1/2 tsp black pepper for 2 minutes more. The vegetables should be soft and translucent, not browned, and the garlic should smell fragrant.

Stir in the 1/2 cup hot sauce and 4 cups rotisserie chicken (chopped), then cook for 2 to 3 minutes and set the mixture aside. The buffalo coating should lightly cling to the chicken and look a bit thicker than when it first went in.

In the same skillet, melt 2 tbsp butter over medium heat, whisk in the 2 tbsp all purpose flour, and keep whisking for 1 minute. Stir constantly so the flour loses its raw taste without taking on much color.

Pour in the 2 1/2 cups half and half, whisk until smooth, then add the 1/4 cup hot sauce and 1 tbsp dijon mustard and lower the heat to a simmer for 4 minutes. The sauce is ready for cheese when it thickens enough to coat the back of a spoon.

Stir in the 1 cup sour cream, 2 cups shredded cheddar cheese, and 2 cups shredded mozzarella cheese until fully melted. Keep the heat low and stop when the sauce looks smooth, glossy, and creamy.

Transfer the macaroni to a greased 13x9-inch casserole dish, add the chicken mixture, pour the cheese sauce over the top, and stir everything together well. Make sure the pasta is fully coated so the casserole bakes up creamy from edge to center.

Cover the dish with aluminum foil and bake for 40 minutes, then uncover, add the crumb topping, and bake 5 minutes more. The top should turn golden and crunchy while the filling bubbles around the edges.