In a small bowl, combine the blueberry preserves and cornstarch. Stir until well combined and set aside.
Unroll the refrigerated pie crusts on a lightly floured surface. Roll out gently if you prefer thinner crusts, about 1/8-inch thickness.
Cut the rolled-out pie crust into rectangles approximately 3x4 inches. You should have about 8 rectangles.
Lay half the rectangles on a parchment-lined baking sheet. Spoon about 1 tablespoon of the blueberry filling onto the center of each, leaving a border.
Place remaining rectangles on top as lids and press edges with a fork to seal them tightly.
Cut a few small slits in the top of each pastry to allow steam to escape during baking.
Beat the large egg with a splash of water or milk. Brush the egg wash over the tops of the Pop Tarts.
Preheat oven to 375°F (190°C) and bake for 20-25 minutes until golden brown.
While the Pop Tarts cool, prepare icing by mixing powdered sugar, blueberry preserves, milk, and vanilla extract. Adjust consistency as needed.
Drizzle the icing over the cooled Pop Tarts. Let set for a few minutes before serving.