Add the vodka. WARNING: the pan can flame (ours didn't). Bring to a boil and cook until the liquid reduces by half. Add the cream and cook 5 more minutes.
Add the marinara or tomato sauce. Bring to a boil, then turn the heat down to low to keep warm.
Boil the penne pasta in hot salted water. Drain, then add to the warm sauce. Once the pasta is well coated in the sauce, pour onto a serving platter or a large pasta bowl. Top with fresh Parmesan cheese and chopped parsley. Serves 6.