Preheat oven to 350 degrees
In a 6-quart pot, bring 6-8 cups of water to boil over high heat
Combine all the grated cheese keeping the 2 ounces aside for topping
In a 3-quart pot, melt 3 tablespoons butter over medium-high heat. Once melted, add garlic and cook until fragrant, about 30 seconds
Add flour and whisk together continuously for 3 minutes. Do not let the roux brown
Whisk in one cup of milk and simmer until thickened
Slowly whisk in the 2 cups of milk. If the mixture is very thick, add more milk until desired consistency. Bring to a simmer, then reduce heat to medium
Meanwhile, add a generous amount of salt to the boiling water in the 6-quart pot, add pasta while stirring. Cook until just al dente per package instruction.
Whisk the cream cheese and Velveeta in 3 patches into the milk mixture until smooth.
Remove from cheese mixture from heat, and in small batches, add the remaining grated cheese, minus the amount set aside. Whisk after each addition until cheese is melted. If needed, place the pot back on the heat briefly.
Stir in the buffalo sauce, per your liking
Drain pasta and transfer to a large bowl and chicken.
Pour the cheese sauce into the pasta and chicken. (it may seem like there is too much sauce)
Transfer into 8x8 baking pan or larger, sprayed with cooking spray, top with remaining shredded cheese, blue cheese crumbles (if using), and butter
Place on a sheet pan and bake for 30 minutes or until bubbling and all the cheese has melted.
Drizzle with ranch dressing if using.
Let rest for 15 to 20 minutes before serving.