Preheat oven to 350 degrees
In a medium to large pot, over medium heat, cook the butter until melted and then whisk in the flour until pasty, about 1-2 minutes.
Whisk in the chicken stock and milk.
Cook over medium high heat until bubbly, about 5-7 minutes. Whisk as it cooks to keep milk from burning to the bottom of the pan
Whisk in granulated garlic, onion powder, salt, and hot sauce. Cook until hot and just starting to bubble, not boiling though.
Remove from heat and stir in both cheeses. Stir until smooth, about 3 minutes.
Stir in the shredded chicken.
Taste for salt.
Place pasta in an 9 x 13 baking pan coated with cooking spray.
Pour Buffalo chicken sauce over pasta and then stir until it is all evenly coated.
Cover with foil and bake for 50 minutes.
Remove foil and bake for another 5-10 minutes, or until hot and bubbly.
Remove from the oven and then let it sit for about 5 minutes and serve hot.