Brush half of the puff pastry rectangles with egg wash and sprinkle generously with demerara sugar, pressing lightly into dough to adhere - these will be your bottom crusts. Flip dough pieces over to reveal non-sugared side.
Place a couple of tablespoons of cherry filling into the center of the bottom puff pastry pieces. Make sure to leave some space around the edges so there's room to seal them.
Brush all 4 edges of the filled puff pastry pieces with egg wash. Place remaining puff pastry rectangles on top of filled pastry, aligning the edges as much as possible. Press along all sides to adhere. Crimp the edges with a fork to seal shut.
Brush each hand pie with egg wash and sprinkle generously with demerara sugar.
Use a sharp paring knife or kitchen scissors to cut a few slits in the top of each hand pie. Place pies in the fridge to chill for at least 20 minutes. At the end of the chilling period, preheat oven to 400° F.
Bake hand pies until golden brown all over and crispy on the bottom, about 25-30 minutes. The cherry filling should be bubbling out through the slits in the top crust.
Allow to cool and top with ice cream or fresh whipped cream for the ultimate summer treat!