Mix the cornstarch and water together in a small bowl until combined and milky. Set aside. In a medium saucepan, combine the cherries, sugar, lemon juice, and salt. Stirring occasionally with a rubber spatula or wooden spoon, cook over medium heat until the cherries begin to release their juices—about 4-6 minutes. Stir in the cornstarch mixture, then bring to a boil while stirring often. Once boiling, let boil for 1 minute. Remove from heat then stir in the vanilla. Allow to cool completely at room temperature. The filling can be made up to 5 days in advance and stored in the refrigerator.

Line a large baking sheet with parchment paper or a silicone baking mat. On a lightly floured surface, lay out one pastry sheet. Using a pizza cutter or sharp knife, cut sheet into 6 rectangles. 3 of these rectangles will be the bottom pastry pie crust, 3 will be the top. Transfer the 3 bottom rectangles to the prepared baking sheet. (Make sure there will room for all 6 pastry pies—if not, use a second baking sheet.) Using a pastry brush, brush the edges of the bottom rectangles with the egg wash. Spoon 2-3 Tablespoons of cooled cherry mixture onto each of the 3 bottom rectangles. Place the top rectangles over the cherry filling, lining up the edges as best you can. Crimp the edges with a fork to seal shut. Repeat this entire step with 2nd pastry sheet so you have a total of 6 pastry pies. Slice 2-3 slits in the tops for air vents (use a super sharp knife to do that). Use my photos in this post to help guide you in this step.

Brush the tops of the pastry pies with egg wash, then sprinkle with coarse sugar. Chill the pies in the refrigerator for at least 25 minutes or up to 1 day.

During the last few minutes of chill time, preheat oven to 375°F (190°C).

Bake the pastry pies for 30-35 minutes, or until the tops are golden brown. Make sure to rotate the baking sheet once or twice during bake time. Remove pies from the oven and allow to cool on the pan until ready to handle and serve. Wrap each leftover pastry pie up tightly in plastic wrap and store in the refrigerator for up to 5 days.