Heat oil or butter in a large skillet over medium heat. Add garlic and red pepper flakes, if using, and cook, stirring often, until fragrant, about 1 minute.

Pour in broth, cream, and 1/2 tsp salt, then add penne. Bring to a boil, reduce to a simmer, and cook 9–10 minutes, stirring occasionally, until pasta is tender and liquid thickens.

Remove from heat. Stir in Parmesan and black pepper until cheese melts and sauce is creamy. Taste and add more salt if needed.

Toss again, then serve immediately in warm bowls with extra Parmesan and black pepper on top.