Pour in the heavy cream and bring to a gentle simmer. Stir in the grated Parmesan cheese, salt, black pepper, and red pepper flakes (if using). Mix until the cheese is melted and the sauce is smooth.
Add the cooked penne pasta to the sauce and toss to coat evenly. Cook for an additional 1-2 minutes to heat through.
Remove from heat and garnish with chopped fresh parsley before serving.