Preheat your oven to 375°F (190°C) and lightly grease a 2-quart baking dish. This is your warm-up phase to get everything ready for a quick bake.
Cook the pasta in salted boiling water just until al dente, about 6–8 minutes depending on the shape. Reserve 1/2 cup of pasta water, then drain. The goal is tender pasta that still holds its bite, because it will continue to cook in the casserole.
In a large bowl, whisk together the buffalo sauce, ranch dressing, and sour cream until smooth. If you’d like a milder sauce, blend in a bit of the reserved pasta water to loosen the mix a touch.
Stir the shredded chicken and 1/2 cup of the mozzarella into the sauce mixture. Add the hot pasta and toss to coat evenly. If the mixture seems a little dry, splash in a tablespoon or two of the reserved pasta water or a splash of milk to loosen it.
Season with a pinch of salt and pepper, then fold in half of the chopped celery or green onions. The fresh crunch is a bright counterpoint to the creamy, spicy base.
Transfer the mixture to the prepared baking dish. Sprinkle the remaining mozzarella and the parmesan evenly on top. If you love extra heat, you can add a few splashes of buffalo sauce on top as well.
Bake in the oven for 20–25 minutes, until the sauce is bubbling and the cheese is melted and lightly golden. If you like a crisper top, turn on the broiler for the last 1–2 minutes, watching closely to prevent burning.
Remove from the oven and let the casserole rest for 5–7 minutes. This helps the flavors settle and makes it easier to slice into generous portions.
Garnish with the remaining celery or green onions and, if you’re a fan, blue cheese crumbles. Serve warm with a simple side salad or crusty bread to soak up every bit of that tangy, creamy sauce.