Heat 2 tablespoons butter in a large pot over medium-high heat. Add the cubed chicken and 2 teaspoons Cajun seasoning and cook, stirring occasionally, until the chicken is cooked through, about 5 minutes. Remove the chicken from the pot and set aside.
Reduce heat to medium. Add the sliced onion, chopped poblano peppers, chopped garlic, and 3 to 4 teaspoons Cajun seasoning to the pot. Cook, stirring, until the vegetables are softened, about 4–6 minutes.
Add the short-cut pasta, chopped parsley, 2 cups water, and a pinch of chili flakes to the pot. Bring to a simmer and cook, stirring often, until the pasta is al dente and most of the water is absorbed, about 8 minutes (follow package timing as needed).
Stir in 2 cups milk, 2 ounces cream cheese, 1/2 cup ranch dressing, and 1 cup grated Parmesan. Cook over low heat, stirring, until the sauce is smooth and creamy.
Return the cooked chicken to the pot and gently fold it into the pasta until heated through. Adjust seasoning with salt, pepper, or more Cajun seasoning if desired.
Serve immediately, garnished with extra Parmesan and parsley if desired.