Prepare the Blueberry Pie Filling:
In a medium saucepan, melt the butter over medium heat.
Add the fresh blueberries, water, sugar, cornstarch, lemon juice, cinnamon, and salt. Stir to combine all the ingredients.
Bring the mixture to a gentle boil over medium to medium-high heat. Lower the heat and let it simmer for about 1 minute until thickened. The mixture should be quite thick.
Set aside the filling to cool slightly before assembling the cookies.
Assemble the Blueberry Pie Cookies:
Roll out the pie crust on a lightly floured surface to about 1/8 to 1/4 inch thick.
Use a 2 1/2 to 3-inch round cookie cutter or biscuit cutter to cut out as many rounds as possible from the dough. You’ll need two rounds for each cookie.
Transfer half of the rounds to a baking sheet, and brush the edges with a small amount of water to help seal them after filling.
Place about 1 1/2 teaspoons of the cooled blueberry filling in the center of each round. Be careful not to overfill.
Top each filled round with one of the remaining rounds. Press the edges together with your fingers to seal the cookies, and then use a fork to crimp the edges for a decorative finish.
Prepare the egg wash by lightly beating the egg and 1/4 teaspoon of water together.
Brush the tops of each pie cookie with the egg wash, ensuring the entire surface is covered. This will give the cookies a nice golden color.
Sprinkle the tops with sanding sugar or granulated sugar if desired.
Use a small knife to cut slits in the top of each pie cookie to allow steam to escape during baking.
Bake the Blueberry Pie Cookies:
Preheat your oven to 350°F (175°C).
Bake the pie cookies for about 15 to 20 minutes, or until the crust is golden brown.
Once baked, remove the cookies from the oven and transfer them to a wire rack to cool.