In a medium saucepan, melt 2 tablespoons of unsalted butter over medium heat. Add in 1 cup of fresh blueberries, 2 tablespoons of water, ½ cup of granulated sugar, 2 tablespoons of cornstarch, 1 tablespoon of lemon juice, ¼ teaspoon of ground cinnamon, and a pinch of salt. Stir well, bring to gentle boil, lower heat, and simmer for about 1 minute until thickened. Let filling cool.

Roll out one prepared pie crust on a floured surface to a thickness of about 1/8 to 1/4 inch. Cut out circles using a round cutter, aiming for about 4 inches in diameter. Moisten edges of half the circles, spoon 1.5 teaspoons of cooled blueberry filling into the center of each, place another round on top, pressing edges together to seal.

Preheat your oven to 350°F (175°C). Place the assembled cookies on a parchment-lined baking sheet. Brush tops with beaten egg for a golden color. Sprinkle sanding sugar on top if desired.

Bake the cookies for 15 to 20 minutes until golden brown. Remove from oven and let them cool on the baking sheet for a few minutes.

Transfer warm cookies to a wire rack to cool completely. Serve for any occasion or enjoy with coffee.