In a large deep skillet or Dutch oven, combine the penne pasta, whole milk, chicken broth, unsalted butter, minced garlic, salt, black pepper, and crushed red pepper flakes.
Bring the mixture to a gentle boil over medium-high heat, stirring frequently to prevent the pasta from sticking to the bottom.
Reduce the heat to medium-low and simmer uncovered, stirring occasionally, until the pasta is tender and most of the liquid has been absorbed, about 15 to 18 minutes.
Remove the skillet from heat and stir in the freshly grated Parmesan cheese until melted and the sauce is creamy.
Sprinkle chopped fresh parsley over the top and serve immediately.