Preheat your oven to 400°F (200°C).

In a bowl, combine the flour and kosher salt. Add the cold butter pieces and mix until the mixture resembles coarse crumbs.

Stir in the white vinegar and cold water. Mix until a dough forms. Be careful not to overwork it; the dough should be slightly crumbly but cohesive.

Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.

Roll out the chilled dough on a floured surface to about 1/4 inch thick. Cut out shapes using a cookie cutter or a knife.

Place the cut pastry shapes onto a lined baking sheet.

Place about a teaspoon of jam in the center of each pastry shape. Be careful not to overfill.

Beat the egg in a small bowl and brush the edges of each pastry with it.

Bake the tarts in the preheated oven for 15-20 minutes or until they are golden brown.

Once baked, let the tarts cool slightly on a wire rack. Dust them with powdered sugar before serving.