Preheat oven to 375°F and grease a 9x13-inch baking dish.
Cook bacon in skillet until crispy, then crumble and set aside.
In same skillet, cook chicken in bacon drippings with half the ranch mix until golden.
Boil pasta until al dente; drain well.
Whisk sour cream, mayonnaise, milk, remaining ranch mix, garlic powder, and pepper.
In a large bowl, combine pasta, chicken, and most of the bacon.
Add ranch sauce and 1.5 cups cheddar; mix until coated.
Transfer to baking dish, top with remaining cheese and bacon.
Cover with foil and bake 25 minutes, then uncover and bake 15–20 more until bubbly and golden.
Rest 5 minutes before serving; garnish with parsley.