In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat. Once melted, add 1 cup of fresh blueberries, 2 tablespoons of water, 1/4 cup of granulated sugar, 2 tablespoons of cornstarch, 1 tablespoon of lemon juice, 1/2 teaspoon of ground cinnamon, and a pinch of salt. Stir continuously until the mixture thickens, approximately 1 minute, then remove from heat and allow to cool slightly.

Roll out a prepared pie crust on a clean, floured surface until it's 1/8 to 1/4 inch thick. Use a round cutter that measures about 2.5 to 3 inches to cut out circles from the dough. Reserve half for topping later.

On a baking sheet lined with parchment paper, place half of the pie crust rounds. Moisten the edges of each round with a little water to help seal them later. Spoon 1.5 teaspoons of the cooled blueberry filling into the center of each round. Top with the remaining crust rounds, pressing down gently to seal.

Preheat the oven to 350°F (175°C). While it heats, prepare an egg wash by whisking together 1 large egg with a splash of water to thin. Brush the tops of each assembled Blueberry Pie Cookie with the egg wash, then sprinkle with sanding sugar. Bake for 15 to 20 minutes or until golden brown and crisp.