Bring a large pot of salted water to boil. Boil the penne according to package directions. Drain and set aside.
While the pasta is cooking, melt butter in a large saucepan over medium heat. When the butter is melted, add the garlic and stir until fragrant, about 1 minute.
Whisk in the flour and cook for a few minutes, or until the mixture is slightly thickened and puffed.
Lower the heat to simmer and slowly whisk in the warm milk a little bit at a time. Whisk continuously until all the milk is added. Turn the heat back up to medium and continue to cook and whisk until the mixture is thickened and coats the back of the spoon.
Stir in the cheeses until melted. Remove from heat, then stir in the hot sauce, dressing, and onion powder. Season to taste with salt and pepper.
In a large bowl or the pot where you cooked the noodles, stir together the pasta, cheese sauce, chicken, and green onions. Drizzle with additional hot sauce and blue cheese crumbles, if desired.