In a medium saucepan over medium heat, melt 2 tablespoons of butter. Add 1 cup of fresh blueberries, 2 tablespoons of water, ½ cup of granulated sugar, 1 tablespoon of cornstarch, 1 tablespoon of lemon juice, a pinch of ground cinnamon, and a pinch of salt. Stir the mixture frequently for about 1-2 minutes until it thickens and starts bubbling, then remove it from heat and allow it to cool slightly.

Roll out your prepared pie crust on a floured surface to about 1/8 to 1/4 inch thickness. Use a cookie cutter to cut out rounds, as many as possible. Moisten the edges of the rounds with a little water. Spoon a small amount of the blueberry filling onto the center of half of the rounds, then top these with the remaining rounds. Press the edges together to seal and crimp them with a fork for a decorative touch.

In a small bowl, whisk together 1 egg and 1 tablespoon of water until well combined. Use a pastry brush to apply this egg wash generously over the tops of your assembled cookies. Sprinkle sanding sugar on top if you desire a little sparkle. Make 2 or 3 small slits in the tops of each cookie to allow steam to escape while baking.

Preheat your oven to 350°F (175°C). Arrange the cookies on a lined baking sheet, leaving space between each one. Bake for 15-20 minutes, or until they turn golden brown and the filling begins to bubble through the slits. Keep an eye on them towards the end to achieve that perfect, warm golden color without burning.

Once baked, remove the Blueberry Pie Cookies from the oven and transfer them to a wire rack. Allow them to cool at room temperature for about 10-15 minutes, making it easier to handle and enjoy these delightful treats while preserving their warm, gooey blueberry filling.