Preheat oven to 375°F. Line a baking sheet with parchment paper.
Add flour, sugar, and salt to a large food processor. Add the cold, cubed butter and shortening, then pulse until the butter and shortening are pretty well cut into the flour leaving only pea-sized pieces.
Stream in the cold water while pulsing the crust mixture until it is evenly moistened and starting to clump together around the blades of the food processor. The dough shouldn't be as shaggy as a typical pie crust dough.
Divide the dough into two evenly-sized pieces and flatten them into squares. Wrap in plastic wrap and chill in the fridge for 30-60 minutes before proceeding. You can even pop them into the freezer for 15-20 minutes to speed up the process.
Unwrap the chilled pastry dough and dust on both sides with some extra flour. I like to roll out the dough onto a large sheet of parchment paper so I don't have to use too much flour on the counter to prevent the dough from sticking. Roll very thin (about ⅛" thickness) into a 9x12-inch rectangle, then use a pizza cutter or sharp knife and a ruler to cut the dough into nine 3x4-inch rectangles.
Brush the surface of each rectangular pastry with egg wash. Top with 1 heaping tablespoon of filling in the center of each rectangle. Evenly spread the filling to about ½-inch from the edges of the pastry.
Add the top pastry crust, matching up the edges. Crimp the edges of each pop-tart with a fork to seal. Brush the tops of the pastries with the remaining egg wash and use a toothpick to poke 9-12 holes in the top of each pop-tart so the filling can vent as they bake.
Bake for about 25-28 minutes until golden brown. Remove from oven and cool on a wire rack.
Combine powdered sugar, corn syrup, cinnamon, vanilla extract, and milk in a medium bowl. Whisk well until smooth. The glaze shouldn't be too thick or too thin. Add more powdered sugar or more milk to reach the desired consistency for your glaze.
Spoon about 1 tablespoon of the glaze over each pop-tart. Let the glaze set up for 3-4 hours before enjoying.