Cook the penne in salted boiling water until al dente. Reserve 1/2 cup pasta water, then drain.
Pat the chicken dry, slice each breast in half horizontally, and rub with 1 tbsp Cajun seasoning and the salt.
Heat the olive oil in a large skillet over medium-high. Cook the chicken 5–6 minutes per side until cooked through. Rest 5 minutes, then slice.
Lower the heat to medium. Melt the butter, then cook the onion and bell pepper until soft. Add the garlic for the last minute.
Pour in the broth and scrape up the browned bits. Stir in the cream, remaining Cajun seasoning, and smoked paprika. Simmer 3–4 minutes.
Turn off the heat and stir in the Parmesan. Loosen with pasta water if needed.
Toss in the pasta and chicken, taste for seasoning, and top with parsley.