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, about 30 seconds.
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 done.
Turn the heat to low, then stir in the cheddar, Monterey Jack and cream cheeses, along with 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.
Drizzle the remaining 2 tablespoons of buffalo sauce over the mac and cheese. Add the reserved chopped chicken, celery and green onions on top, then serve.