Step 1: Make the Dough

1. In a large mixing bowl, combine the flour, sugar, and salt. Add the cold, cubed butter and use a pastry cutter or your fingers to cut the butter into the flour until the mixture resembles coarse crumbs.
2. Gradually add the ice water, one tablespoon at a time, mixing gently until the dough comes together. Be careful not to overmix.
3. Divide the dough into two equal portions, flatten into discs, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Step 2: Prepare the Blueberry Filling

1. In a medium saucepan over medium heat, combine the blueberries, sugar, cornstarch, and lemon juice. Stir well to combine.
2. Cook for 5–7 minutes, stirring frequently, until the blueberries release their juices and the mixture thickens. Remove from heat and stir in the vanilla extract. Let the filling cool completely.

Step 3: Roll Out the Dough

1. Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
2. On a lightly floured surface, roll out one disc of dough to about 1/8-inch thickness. Use a round cookie cutter (3–4 inches in diameter) to cut out circles. Repeat with the second disc of dough.

Step 4: Assemble the Cookies

1. Place half of the dough circles on the prepared baking sheet. Spoon about 1 tablespoon of the cooled blueberry filling into the center of each circle, leaving a small border around the edges.
2. Brush the edges of the dough with the beaten egg wash, then top each with another dough circle. Press the edges together with a fork to seal.
3. Use a small knife to cut slits or a decorative design into the top of each cookie for ventilation. Brush the tops with egg wash and sprinkle with sugar.

Step 5: Bake and Cool

1. Bake the cookies in the preheated oven for 18–20 minutes, or until the crust is golden brown.
2. Remove from the oven and allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
3. Dust with powdered sugar if desired and serve.