Step 1: Make the Blueberry Filling

1. In a small saucepan, combine blueberries, water, and sugar over medium heat.
2. Stir occasionally until the blueberries start to soften and burst.
3. Mix cornstarch with 1 tablespoon of water to create a slurry, then stir it into the blueberry mixture.
4. Continue cooking until the mixture thickens (about 5 minutes).
5. Remove from heat and stir in butter and vanilla extract. Set aside to cool.

Step 2: Prepare the Pie Dough

1. In a bowl, mix flour and salt. Add cubed butter and use a pastry cutter or fork to blend until it resembles coarse crumbs.
2. Slowly add ice water, 1 tablespoon at a time, mixing until the dough comes together.
3. Wrap in plastic wrap and chill for 30 minutes.

Step 3: Assemble the Cookies

1. Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. Roll out the chilled pie dough on a floured surface to ¼-inch thickness.
3. Use a round cookie cutter (or a glass) to cut out circles for the cookies.
4. Place half of the circles on the baking sheet and spoon a small amount of blueberry filling in the center.
5. Place the remaining pie crust circles on top and gently press the edges to seal.
6. Crimp edges with a fork and brush with egg wash for a golden finish.
7. Sprinkle with coarse sugar (optional) for extra crunch.

Step 4: Bake & Enjoy!

1. Bake in preheated oven for 15-18 minutes, or until the cookies are golden brown.
2. Allow to cool slightly before serving.
3. Enjoy warm or at room temperature!