Let the sauce boil and thicken until the penne is done. If the sauce starts to stick, reduce the heat to medium. Pour the sauce into the pot with the penne and toss until the penne is coated with sauce. Serve at once. Anchovy paste is sold in tubes, most often in the supermarket's imported-food section.
Tomato paste is also available in tubes alongside foods imported from Italy.
If you use canned tomato paste, refrigerate the leftover paste for later use in an airtight container for up to 5 days. Parsley adds color and zing, but the recipe is perfectly good without it.