In a mixing bowl, combine flour, sugar, and salt.
Add cold butter and mix until crumbly.
Stir in the sourdough starter.
Add cold water gradually until a dough forms.
Knead the dough lightly and wrap it in plastic.
Chill the dough for at least 30 minutes.
Preheat the oven to 375°F (190°C).
Roll out the dough on a floured surface to about 1/8 inch thick.
Cut out rectangles and place half on a baking sheet.
Fill the center of each rectangle with jam, then top with another rectangle.
Seal the edges with a fork and cut slits on top for steam.
Bake for 20-25 minutes until golden brown.
Dust with powdered sugar before serving.