Preheat oven to 350ºF. Lightly spray 9×13-inch pan with cooking spray.: As the oven warms to 350ºF , you will notice the steady hum and the small burst of warmth when you open the door, which signals the environment is ready for baking. Preparing the pan early prevents delays when your mixture is ready to go. A light coating of spray ensures the casserole releases cleanly and helps edges brown evenly, avoiding sticking which makes dishing out slices messy. A common misstep is skipping the coating and then struggling to lift serving portions, so take a moment to spray and level the pan before you start assembling the pasta. The warm oven also helps the casserole begin to bubble promptly once it goes in.
Prepare pasta according to package directions; drain and set aside.: While the water heats, you will hear the kettle like whistle of a rolling boil and see the surface shimmer as you add the penne pasta . Cook until it is just tender to the bite, al dente, because the pasta will continue to soften in the oven; this prevents a mushy final texture. Drain thoroughly and give it a gentle shake to remove excess water so the sauce clings rather than pools. A frequent error is overcooking, which results in limp pasta that cannot hold the creamy sauce during baking.
In a large bowl, stir together alfredo sauce, sour cream, ricotta, ranch seasoning, chicken, hot sauce, eggs, parsley, and pasta.: When you combine the alfredo sauce , sour cream , and ricotta cheese , you will notice the mix becoming glossy and thick, with a gentle tang from the sour cream . Fold in the shredded chicken and the buffalo hot sauce until the color is evenly distributed, which helps each bite carry both creaminess and heat. The lightly beaten eggs act as a binder, smoothing the texture and helping the casserole hold its shape after baking. Stir until every piece of pasta is coated, and sprinkle in the chopped parsley so it warms and releases a fresh aroma. Avoid overmixing which can break down the noodles and make the mixture gluey.
Spread mixture into a lightly greased 13- x 9-inch baking dish. Sprinkle evenly with cheddar cheese and mozzarella cheese.: As you transfer the cheesy, saucy pasta into the prepared dish, spread it out so the surface is level; this encourages even baking and consistent browning. The casserole should present a glossy, creamy layer before the cheeses go on top, and when you scatter the cheddar cheese and mozzarella cheese they form a uniform blanket that will melt and seize into a golden crust. Pay attention to the edges where bubbling often begins first, and keep the cheese distribution even to prevent greasy pools. A common mistake is piling cheese in one area, which results in uneven melting and texture differences across the dish.
Bake uncovered for 30 minutes or until bubbly.: In the oven, the casserole will shift from glossy to actively bubbling, and the top will start to color, giving off a rich, toasty aroma that signals readiness. Look for steady bubbling around the edges and a light golden brown on the cheese as your visual cues; the sound is a low simmer from the sauce releasing steam. Baking uncovered allows moisture to evaporate which concentrates flavor and creates that pleasant crust. If you bake too long, the top can dry out and the interior may become firmer than intended, so check near the end of the time and tent with foil only if the crust is browning too quickly.