In a large deep skillet or wide pot, combine the penne pasta, whole milk, heavy cream, minced garlic, salt, black pepper, and crushed red pepper flakes.
Place the pot over medium heat and bring the mixture to a gentle simmer, stirring frequently to prevent the milk from scorching.
Cook uncovered, stirring often, until the pasta is tender and most of the liquid is absorbed, about 15 minutes.
Reduce the heat to low and add the unsalted butter. Stir until melted and incorporated.
Gradually add the grated Parmesan cheese, stirring continuously until the cheese is fully melted and the sauce is creamy.
Taste and adjust seasoning with additional salt and pepper if needed.
Remove from heat and sprinkle with chopped fresh parsley before serving.