In a large mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in cold butter until the mixture resembles coarse crumbs. Gradually add ice water until a soft dough forms. Wrap in plastic and refrigerate for at least 30 minutes.

In a separate bowl, mix fresh blueberries, granulated sugar, and vanilla extract. Lightly mash some blueberries to release juices. Set aside to let flavors meld.

Once chilled, divide the dough in half. Roll out one half to 1/8 inch thick and cut into rectangles measuring 3x4 inches. Transfer half onto a parchment-lined baking sheet.

Spoon blueberry filling onto the center of each rectangle, leaving a half-inch border. Place remaining rectangles on top, pressing to seal edges. Crimp with a fork and cut slits for steam to escape.

Preheat oven to 375°F (190°C). Brush tops with egg wash and bake for 20-25 minutes until golden brown and flaky.

Mix powdered sugar with milk or water until smooth to create icing. Drizzle over warm pop tarts before serving.