Preheat your oven to 350°F and grease a 9x13-inch baking dish with butter or cooking spray. Cook the spaghetti according to package directions until just al dente, about 8 to 9 minutes. The pasta should still have a slight firmness when you bite into it. Drain well and set aside.

In a large mixing bowl, combine the softened cream cheese, cream of chicken soup, sour cream, ranch seasoning mix, garlic powder, and black pepper. Use a whisk or spoon to blend everything until smooth, or use a hand mixer on low speed.

Fold in the shredded chicken, 1½ cups of the cheddar cheese (reserve the remaining ½ cup for topping), all of the Parmesan cheese, and the cooked spaghetti. Mix gently but thoroughly so every strand of pasta is coated in the creamy sauce.

Pour the mixture into your prepared baking dish and spread it evenly. Top with the remaining ½ cup of cheddar cheese and sprinkle the crumbled bacon over the top.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and continue baking for another 10 to 15 minutes until the cheese is bubbly and starting to turn golden brown around the edges.

Let the casserole rest for 5 to 10 minutes before serving. This resting time allows the sauce to thicken slightly. Garnish with fresh parsley if desired and serve warm.