In a large bowl, whisk together flour, sugar, and salt. Cut in cold butter until the mixture resembles coarse crumbs.

Add cold water gradually until the dough comes together. Divide in half, shape into discs, wrap, and chill for 30 minutes.

Meanwhile, make the blueberry filling by combining blueberries, sugar, lemon juice, and cornstarch in a saucepan. Cook over medium heat until thickened. Cool completely.

Preheat oven to 375°F (190°C) and line a baking sheet with parchment paper.

Roll out one dough disc to 1/8-inch thick. Cut into equal rectangles (about 3×4 inches).

Place half of the rectangles on the baking sheet. Add a spoonful of blueberry filling to the center of each, leaving a border.

Brush edges with egg wash, place another rectangle on top, and seal edges with a fork. Poke holes on top to vent steam.

Brush tops with egg wash and bake for 20–25 minutes or until golden brown. Cool completely before glazing.

Whisk powdered sugar, milk, and vanilla until smooth. Spread glaze over cooled pop tarts and allow to set before serving.