In a saucepan over medium heat, melt the butter. Add blueberries, water, sugar, cornstarch, lemon juice, cinnamon, and salt. Bring to boil then reduce heat and simmer for 1 minute. Remove from heat and let cool completely.

On a lightly floured surface, roll out your pie crust to 1/8 to 1/4 inch thick. Cut out circles using a round cutter. Gather scraps, re-roll, and continue cutting until used up.

Preheat oven to 350℉. Line a baking sheet with parchment paper. Place half of the pie crust rounds on the sheet, brush edges lightly with water. Spoon 1.5 teaspoons of filling onto each round. Place another round on top, press edges to seal, and crimp with a fork.

In a small bowl, whisk together the egg and water for an egg wash. Brush over the tops of each cookie and sprinkle with sugar if desired.

Bake for 15 to 20 minutes until golden brown and flaky. Allow to cool on a wire rack before serving.