Preheat your oven to 400ºF and line a baking tray with parchment paper. Set the tray aside for now.
Add the cornstarch, sugar, and salt to a medium sized pot and stir together.
Add the blueberries to the pot and toss them in the sugar mixture.
Place the pot over medium heat and bring the mixture to a simmer, stirring occasionally. Stir and simmer for about 5 minutes or until the sauce begins to thicken. Remove from the stove and let cool.
Add the lemon zest, lemon extract and vanilla extract to the cooked blueberries and stir.
Roll the pie crusts on a floured work surface, making them 14x14” squares. This can be tricky if your pie crusts were circular, but just work on rolling the edges and working the dough from the center outward toward the corners to make it square.
Use a pizza cutter or sharp knife to cut the dough into 3 ½ x 3 ½ inch squares. You should get about 16 squares from each pie crust.
Divide the filling between 8 squares of dough, scooping the filling into the center of the square. You should be able to add about a tablespoon of blueberry filling to each square.
Brush a little of the whisked egg around the edges of the squares that have the blueberry filling.
Top each square with a second square of dough, laying the second piece right on top of the filling. Press the edges of the top dough on to the bottom, sealing it closed. You can even use a fork to go around the edges of the square and really seal them well.
Place the stuffed dough squares on the prepared baking tray then cut a slit about 1 inch long in the top of each square.
Brush the pastries with the remaining egg white then sprinkle each with a little extra sugar.
Bake the hand pies for 18 minutes. They will be a light golden brown.
Let the pies cool for about 15 minutes before serving.