Bring a large pot of salted water to a boil. Add the 8 ounces of penne pasta and cook according to package instructions until al dente, usually around 9-11 minutes. Drain the pasta and set it aside.
If not already cooked, prepare your chicken by poaching or baking until fully cooked, then shred it with two forks. Cook the turkey bacon in a skillet over medium heat until crisp, then crumble it into bite-sized pieces.
In a large mixing bowl, combine 1 cup ranch dressing, 1 cup milk, 1 teaspoon garlic powder, 1 teaspoon onion powder, and 1/2 teaspoon black pepper. Stir well until the sauce is smooth and evenly blended.
Add the cooked penne pasta, shredded chicken, crumbled turkey bacon, and 1 cup shredded cheddar cheese to the bowl with the ranch sauce. Toss everything gently to coat the pasta and proteins evenly with the creamy ranch mixture.
Transfer the pasta mixture into a greased 9x13-inch baking dish. Spread it out evenly. Sprinkle 1/2 cup sliced green onions over the top, followed by 1/2 cup breadcrumbs for a crunchy finish.
Preheat your oven to 350°F (175°C). Bake the assembled casserole for 25-30 minutes or until the cheese is bubbling and the breadcrumbs turn a beautiful golden brown.
Remove the Ranch Chicken Bacon Penne Bake from the oven and let it cool for 5 minutes before serving. This gives the sauce a chance to thicken up slightly, making it even more delicious.