Preheat and Prepare : Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or a silicone mat for easy cleanup.

Make the Blueberry Filling : In a small saucepan, combine the blueberries, sugar, cornstarch, cinnamon, salt, lemon juice, and water. Cook over medium heat, stirring occasionally, until the mixture thickens (about 5–7 minutes). Remove from heat and let it cool to room temperature.

Prepare the Pie Crust : Roll out your prepared pie crusts on a lightly floured surface. Use a round cookie cutter (3–4 inches in diameter) to cut out circles. Re-roll scraps as needed to maximize the number of circles.

Assemble the Cookies : Place a small spoonful of the blueberry filling in the center of half of the pie crust circles. Dot with a tiny piece of butter. Cover each with a second circle of pie crust, pressing the edges together to seal. Use a fork to crimp the edges for a decorative touch.

Apply Egg Wash : Beat the egg in a small bowl and brush the tops of the cookies with the egg wash. For extra shine and sweetness, sprinkle a pinch of granulated sugar over the tops.

Bake the Cookies : Arrange the cookies on the prepared baking sheet, leaving space between them. Bake in the preheated oven for 15–18 minutes, or until the tops are golden brown. Remove and allow them to cool on a wire rack.

Serve and Enjoy : Serve the cookies warm or at room temperature. Pair with a dollop of whipped cream or a scoop of vanilla ice cream for a decadent touch!