Sauté the base aromatics: In a large, deep skillet or sauté pan, melt 2 tablespoons of butter over medium heat. Add diced onion and celery, and sauté for 2–3 minutes until softened and translucent. Stir occasionally to prevent browning. Add the minced garlic and sauté for another 30 seconds, just until fragrant.
Build the broth: Pour in 3 cups of chicken broth and the entire can of diced tomatoes, including the juice. Stir to combine and bring the mixture to a gentle boil over medium-high heat. This creates a flavorful base that the pasta will cook in, infusing it with savory depth.
Cook the pasta: Add 3½ cups of dry rotini pasta directly into the bubbling liquid. Stir well, reduce heat slightly, and let simmer for 9–10 minutes, or until the pasta is al dente. Stir occasionally to ensure even cooking and prevent sticking. The pasta should absorb much of the broth while retaining a slight bite.
Add the creaminess: Once the pasta is nearly done, reduce heat to low. Stir in cubed cream cheese and mix gently until melted and smooth. Add in the shredded cheddar and mozzarella cheeses, followed by the ranch dressing and Buffalo sauce. Stir until the mixture is fully combined, creamy, and evenly coating the pasta.
Finish with chicken: Fold in the cooked, cubed chicken breast. Let everything simmer together for 2–3 more minutes so the flavors meld and the chicken heats through. Taste and adjust seasoning with extra salt, pepper, or Buffalo sauce as desired.
Serve hot: Spoon the creamy, cheesy Buffalo pasta into bowls and serve immediately. Garnish with extra cheese, green onions, or a drizzle of ranch if desired.
