Preheat your oven to 375°F (190°C). Cook elbow macaroni according to package instructions until al dente; drain.
In a large saucepan, melt butter over medium heat. Whisk in flour until smooth and cook for 1-2 minutes.
Gradually whisk in milk, stirring frequently until the mixture thickens (about 5 minutes). Remove from heat and stir in salt, black pepper, and cayenne pepper.
Add shredded cheeses until melted and smooth. Fold in BBQ chicken and BBQ sauce until well combined.
Mix the cheese sauce with cooked macaroni until evenly coated. Pour into a greased baking dish.
Combine breadcrumbs with melted butter and sprinkle over the mac and cheese. Bake for 20-25 minutes until golden brown.