Preheat the oven to 375°F and spray a 9×13-inch baking pan with nonstick cooking spray.
Cook the penne pasta according to the package directions until al dente. Drain, rinse, and set aside.
In a large mixing bowl, combine the softened cream cheese, ranch seasoning mix, milk, and ¼ cup of the buffalo sauce. Stir until smooth and well combined.
Add the cooked shredded chicken, pasta, and half of each shredded cheese. Stir until evenly coated.
Pour the mixture into the prepared baking dish. Drizzle the remaining buffalo sauce over the top and sprinkle with the remaining shredded cheeses.
Bake for 25–30 minutes, until heated through and the cheese is fully melted.
Remove from the oven and let rest for 10–15 minutes. Top with the sliced green onions and serve.
Notes
Refrigerate the leftovers in an airtight container for up to 5 days.