Bring a large pot of salted water to a boil. Add the penne pasta and cook according to package instructions until al dente, usually about 10-11 minutes. Drain the pasta in a colander and set aside.
In a large mixing bowl, whisk together the heavy cream, grated Parmesan cheese, garlic powder, black pepper, and salt until smooth and combined to form the Alfredo sauce base.
In another bowl, toss the shredded chicken with the Buffalo wing sauce until well coated.
Add the cooked pasta to the Alfredo sauce mixture, gently folding to coat all the noodles. Then fold in the Buffalo chicken and half of the chopped green onions, evenly distributing the flavors.
Transfer the pasta and chicken mixture to your greased oven-safe casserole dish. Sprinkle the shredded mozzarella cheese evenly over the top.
Preheat your oven to 350°F (175°C). Bake the casserole uncovered for 20-25 minutes, or until the cheese is melted and bubbly with golden spots.
Remove from the oven and let it cool for a few minutes. Sprinkle the remaining green onions and fresh parsley on top. Serve warm and enjoy!