In the bowl of a food processor, pulse together flour, salt and sugar. Add butter and shorting then pulse just a few times until butter and shortening are crumbly. Add cold water and pulse until a dough forms.
Divide dough into two equal parts, form into disks about the size of your hand. Cover with plastic wrap and place in fridge for 30 minutes.
While the dough is chilling, prepare filling by mixing together water and cornstarch in a small sauce pan. Stir in preserves then heat on medium until boiling. Reduce heat and simmer for 5 minutes until the filling has thickened.
Bring dough disks out of fridge, using a rolling pin roll out dough into 2 roughly 10x12 inch rectangle about ⅛ inch thick. Use a pizza cutter to trim the edges to make a clean rectangle. Then cut 16 equal sized rectangles. Place ½ of rectangles evenly spaced on a sheet of parchment paper the size of your baking sheet.
Spoon 2 tablespoons filling into the middle of each pastry rectangle.
Cover the prepared rectangles with the other half of the prepared pastry rectangles, making sure to line up the edges. Finger press around the edges to seal. Prick the top of each pop-tart with a toothpick to vent. Press the tines of a fork around each edge to completely seal.
Brush edges with egg wash.
Carefully transfer parchment paper to your baking sheet. Place baking sheet in freezer for one hour.
Preheat oven to 375℉.
Place Frozen baking sheet in preheated oven. Bake for 15 minutes, then rotate sheet in oven for even browning. Bake for a total of 30 minutes.
Cool slightly before transferring to a cooling rack
Completely cool before adding frosting and sprinkles.