Dough*

1. Grate the butter or cut it into small pieces. Put it in the freezer. 
2. Mix flour, salt and sugar in a bowl.
3. Begin working the butter into the flour with a pastry cutter or a pair of forks, but do not incorporate entirely. The dough should still have flecks of butter in it.
4. Add the vinegar and half of the water. Begin mixing the dough. Gradually add the rest of the water.
5. Once the dough has formed, shape it into a disc. Dust your working surface with flour and roll out the dough away from you until it is about 4 mm (⅛  inch) thick.
6. Fold the dough into thirds (like a letter). To do this, fold the top third of the dough over the center and then fold the bottom third over the center. Turn the dough 90 degrees, roll out again and repeat the folds.
7. Cut the dough in half, wrap it in plastic and let chill for 30 minutes.

Assembly and Baking

1. Heat the oven to 220 C (450 F).
2. Roll one half of the dough into a rectangle 20 x 30 cm (8 x 12 inches). Cut into six squares.
3. Diagonally cut the corners of each square towards the center, leaving enough space in the middle to place the jam.
4. Add 1.5-2 tablespoons jam to the center of each square.
5. Fold one corner from each cut piece towards the center of the square. Pierce with a toothpick to hold it in place.
6. Repeat with the second half of the dough.
7. Brush with egg wash (optional) and bake for 12-15 minutes.
8. Remove from oven and let cool. Sprinkle with powdered sugar.