Preheat the oven to 375 degrees F. Coat a 3 quart baking dish with cooking spray.
Bake the chicken fingers according to package directions. Coarsely chop the chicken. Reserve 1/2 cup of chicken pieces for garnish.
Melt the butter in a large pot over medium heat. Add the flour, then whisk until combined.
Pour in the water and whisk until smooth and just thickened.
Add the milk and whisk until combined.
Stir in the pasta, salt, ranch powder and smoked paprika, then bring the mixture to a simmer.
Cook for 10-12 minutes, stirring occasionally, until pasta is al dente.
Turn the heat to low, then stir in the 3 cups of cheddar cheese, the mozzarella and cream cheeses, and 1/3 cup of buffalo sauce. Keep stirring until the sauce is smooth. If you need to thin the sauce, add more milk, 1 tablespoon at a time, until desired consistency is reached.
Stir in the diced chicken.
Pour the pasta mixture into the baking dish. Sprinkle the remaining cup of cheddar cheese over the top.
Bake for 10-15 minutes until cheese is melted and browned.
Drizzle the remaining 2 tablespoons of buffalo sauce over the pasta. Add the reserved chopped chicken, celery and green onions on top, then serve.