The "Pączki" or Polish doughnut is one of the most traditional Polish dessert available since the time of King Augustus III of Poland in the early 18th century. Pączki are deep-fried pieces of dough shaped into flattened spheres and filled with confiture or other sweet filling. They are mostly consumed on "Fat Thursday" or "tlusty czwartek" in Polish - a traditional catholic christian feast marking the last Thursday before the lent.
It has been recorded that 100 million "paczki" are consumed every year just on this one day.
Definitely worth a try while in Krakow.