Preheat your oven to 375°F and spray a 9x13-inch baking dish with cooking spray. Cook the macaroni according to package directions until al dente, then drain and set aside.
In a large pot over medium heat, melt the 4 tablespoons of butter. Whisk in the flour and cook for 1-2 minutes until it's golden and smells nutty.
Gradually pour in the milk while whisking constantly. Continue whisking until the mixture thickens, about 5 minutes.
Reduce heat to low and stir in the cream cheese cubes until melted and smooth. Add 1½ cups of the cheddar cheese, mozzarella, and blue cheese (if using). Stir until everything is melted and combined.
Stir in the buffalo sauce, then add the cooked macaroni and shredded chicken. Mix everything together until well coated.
Pour the mixture into your prepared baking dish. Top with the remaining ½ cup of cheddar cheese.
In a small bowl, mix the panko breadcrumbs with 2 tablespoons of melted butter, then sprinkle over the casserole.
Bake for 25-30 minutes until bubbly and the top is golden brown. Let it cool for 5 minutes, then serve with ranch dressing and celery sticks on the side!