Preheat oven to 350°F (175°C).
Cook the penne pasta according to package instructions until al dente. Drain and set aside.
In a large mixing bowl, combine the cooked shredded chicken, buffalo sauce, Greek yogurt, ranch dressing, garlic powder, onion powder, black pepper, and salt. Mix until well combined.
Add the cooked pasta to the buffalo chicken mixture and stir to coat evenly.
Stir in half of the shredded cheddar cheese and half of the shredded mozzarella cheese.
Lightly grease a 9×13 inch baking dish with butter.
Transfer the pasta mixture into the prepared baking dish and spread evenly.
Sprinkle the remaining cheddar and mozzarella cheese evenly over the top.
Sprinkle chopped celery and green onions over the cheese layer.
Cover the dish tightly with aluminum foil if freezing. To freeze, place the covered dish in the freezer for up to 3 months.
To bake from frozen, preheat oven to 350°F (175°C), remove foil, cover again with fresh foil, and bake for 45 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and golden.
If baking immediately, bake covered for 20 minutes, then uncovered for 10 minutes until cheese is melted and bubbly.
Let the pasta rest for 5 minutes before serving.