Set the oven to 375°F and grease a 3-quart baking dish with cooking spray.
In a large skillet over medium heat, melt the butter and sauté the garlic for 30 seconds. Stir in the flour and cook for 1 minute to form a roux.
Gradually whisk in the milk and heavy cream, cooking the mixture until it thickens slightly while stirring continuously.
Incorporate the Parmesan and ½ cup of the mozzarella into the sauce. Season with salt and pepper to your preference and stir until the cheese is completely melted.
Combine the cooked penne and chicken in a sizable mixing bowl. Pour the sauce over the pasta and stir thoroughly to ensure an even coating.
Transfer the pasta blend to the prepared baking dish. Sprinkle the remaining mozzarella over the top.
Bake without covering for 20 minutes, or until the cheese is golden and the edges are bubbling.
Finish with a sprinkle of chopped parsley before serving.