Mix together the flour, granulated sugar and salt. Cut the cold butter and chilled vegetable shortening into the flour mixture until incorporated into the dough . Measure out 120 mL (1/2 cup) of water, add ice and stir, then remeasure out 120 mL of the ice water.
Take one tablespoon of ice water and mix it into the flour mixture. Keep doing this until the dough starts to form larger pieces and the water is used up. Fold the dough into two equal sized discs. Refrigerate for two hours.
Over the stove on medium heat, combine the blueberries, granulated sugar and lemon juice. Once they are soft, smash the blueberries so they release their juices. Let simmer for 5-10 minutes until thickened. Run the filling through a strainer to discard of the pulp. Reserve one tablespoon of filling for the icing. Let cool.
Roll out one disc of dough into a 9x12 inch rectangle. Cut the dough into 9 equal rectangles, all measuring 4x3 inches. Do this with both discs of dough. Apply an egg wash to one set of 9 poptarts, spread on the filling, then cover the poptart with another rectangle.
Use a fork to crimp the edges and seal them shut. Cover the entirety of the poptart with the egg wash. Poke a few holes in the top so the steam can release. Bake for 25-27 minutes at 350 degrees Fahrenheit until golden brown.
Whisk together the powdered sugar, reserved blueberry filling and whole milk until smooth and combined. Ice the poptarts and enjoy!