Preheat your oven to 200°C (400°F) and line a baking sheet with parchment paper.
Roll out the puff pastry sheet and cut it into 9 equal squares.
Using a sharp knife, make diagonal cuts from each corner of the squares towards the center, leaving a small uncut square in the middle.
Place a small dollop of prune or plum jam in the center of each square.
Take one corner of each square and fold it towards the center, slightly overlapping the dough with the jam.
Repeat with the remaining corners, creating a pinwheel shape.
Brush the Joulutorttu with the beaten egg wash.
Bake in the preheated oven for 15-20 minutes or until the pastries are golden brown and puffed up.
Allow the Joulutorttu to cool slightly before dusting with powdered sugar.
