1. Bring a large pot of salted water to a boil and cook penne until al dente; reserve 0.5 cup pasta water and drain.

2. Pat chicken dry, then toss with Cajun seasoning, salt, and pepper until evenly coated.

3. Heat olive oil and 1 tablespoon butter in a large skillet over medium-high; sear chicken in a single layer until golden and cooked through, 5 to 7 minutes. Transfer to a plate.

4. Reduce heat to medium, add remaining 2 tablespoons butter to the skillet, then stir in garlic; cook until fragrant, 30 seconds.

5. Pour in heavy cream; simmer gently 2 to 3 minutes, scraping browned bits from the pan.

6. Whisk in Parmesan until smooth and creamy; loosen with splashes of reserved pasta water until the sauce lightly coats a spoon.

7. Return chicken and any juices to the skillet; add cooked penne and toss until everything is glossy and well coated.

8. Stir in parsley and red pepper flakes if using; taste and adjust salt, pepper, or Cajun seasoning.

9. Serve hot with extra Parmesan on top.

10. For extra silkiness, rest the pasta 2 minutes off heat before serving.