Boil pasta & prep oven: Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking casserole dish with olive oil or cooking spray. Bring a large pot of salted water to a rolling boil and cook penne pasta until 1 minute shy of al dente (about 9 minutes). Drain and set aside.
Make garlic ranch roux: Melt butter in a large deep skillet or Dutch oven over medium heat. Add minced garlic and cook for 1 minute until fragrant. Sprinkle in flour and whisk constantly for 1 to 2 minutes to eliminate raw flour flavor.
Simmer cream sauce: Gradually whisk in the chicken broth and heavy cream, whisking vigorously to smooth out any lumps. Stir in the dry ranch dressing seasoning packet and black pepper. Bring to a gentle simmer and cook for 3 to 4 minutes until thickened into a glossy, velvety sauce.
Melt cheeses & fold pasta: Lower heat to low. Stir in 1 cup of shredded mozzarella and 1/2 cup of sharp cheddar until completely melted into the sauce. Add the drained penne pasta, shredded chicken, and 3/4 of the crumbled bacon. Gently toss until every pasta tube is generously coated.
Layer in baking dish: Transfer the pasta mixture into the prepared 9x13-inch baking dish, spreading it into an even layer. Top evenly with the remaining 1 cup of mozzarella, 1/2 cup of cheddar, and reserved crispy bacon crumbles.
Bake to bubbly perfection: Bake uncovered at 375°F (190°C) for 18 to 20 minutes until the cheese is completely melted and bubbly. For extra golden blistering, broil on high for 2 minutes (watching closely). Remove from oven, rest for 5 minutes, garnish with fresh parsley, and serve hot!
