Preheat oven to 375°F; spray a 9x13-inch dish with cooking spray. Cook macaroni according to package until al dente; drain and set aside.
Melt 4 tablespoons butter in a large pot over medium; whisk in flour and cook 1-2 minutes until golden and nutty-smelling.
Gradually whisk in milk constantly; whisk about 5 minutes until thickened.
Reduce heat to low; stir in cream cheese until melted and smooth, then 1½ cups cheddar, mozzarella, and optional blue cheese until melted and combined.
Stir in buffalo sauce; add macaroni and chicken and mix to coat well.
Pour into dish; top with remaining ½ cup cheddar.
Mix panko with 2 tablespoons melted butter in a small bowl; sprinkle over casserole.
Bake 25-30 minutes until bubbly and golden brown; cool 5 minutes and serve with ranch and celery sticks.