1. Cut pancetta into 1/4-inch dice. Put in a 5- to 6-quart pan.
2. Trim and discard root ends from leeks, then rinse well and chop.
3. Trim and discard tops, root ends, and bruised spots from fennel, then rinse and chop.
4. Add leeks, fennel, and fennel seed to pancetta and stir often over medium-high heat until limp but not browned, about 5 minutes.
5. Add cream and oysters and their liquor and turn heat to medium-high. Stir often just until soup is hot but not boiling, about 4 minutes.
6. Stir in 1/2 cup parsley, liqueur, and pepper and salt to taste.
7. Ladle into bowls and sprinkle equally with remaining parsley, then add a dot of butter to each.