The traditional Polish filled doughnut associated with Fat Thursday is a pączek.
Pączki are deep-fried pieces of rich, yeast-raised dough filled with ingredients such as rose-petal jam, plum jam, custard, or other sweet fillings. They are commonly finished with powdered sugar, icing, or candied orange peel.
In Poland, pączki are especially linked with Tłusty Czwartek, or Fat Thursday, the Thursday before Ash Wednesday. The day marks the approach of Lent and is traditionally associated with using up rich ingredients before the fasting season.
In English, the singular is often written pączek and the Polish plural is pączki. English-language speakers sometimes use “paczki” as a general plural, but the Polish spelling preserves the diacritic and the original pronunciation.