Firefly Tart is a traditional food from the undergas of Zeelich.
When Twinsen visits the Village of the Wannies, he tastes the tart and saves a slice. Then he finds out that the tart is made of fireflies and passes out. Later, he needs to give the slice to the old man in in the village to get the key to Dark Monk's chapel.
"You don't like this tart, but you must give it to someone who appreciates it more than you."