In a bowl mix together plums, syrup, jam, cinnamon, sugar and salt. Once combined mix in cornstarch and set aside.
Unwrap and unroll puff pastry. Lay flat. Cut each sheet into six relatively equal rectangles giving you a total of 12.
Lay 6 puff pastry rectangles on a baking sheet lined with parchment. On each rectangle spoon a good serving of the plum filling. Do this with all six rectangles. Don't worry if the filling spills over the edge.
Place the remaining pieces of puff pastry over each piece to cover. Crimp the edges with a fork to seal in all the filling. Don't worry if it oozes out. Chill the pies in the refrigerator for 30 minutes.
Preheat your oven to 375 F.
Remove hand pies from refrigerator and brush egg on each one. Make two slits on top to allow steam to escape.
Bake for 25-30 minutes until golden brown.
Remove pies from oven and allow to cool in pan. Enjoy immediately or once cool wrap individually in plastic wrap and store in refrigerator for up to five days.