For the pastry

1. Follow the directions for the all-butter pie crust recipe.  Line two baking sheet pans with silicone baking mats or parchment paper and set aside.
2. Remove one of the the chilled dough discs from the refrigerator and allow to sit at room temperature for 10-15 mintues.  On a lightly floured work surface, roll the dough into a 9 by 12-inch rectangle.  Using a pastry wheel or knife, cut the dough into 9 rectangles, each measuring 3 by 4-inches.
3. Gently transfer the cut dough to one of the prepared baking sheet pans leaving 2-inches between each pastry.  Place the baking sheet pan into the refrigerator and remove the second dough disc.  Repeat the rolling and cutting process with the second dough.  Transfer the rectangles to the second baking sheet pan and place in the refrigerator to chill while preparing the filling.
4. In a small bowl, whisk the cornstarch and water together.  Add the jam and cornstarch mixture to a small saucepan over medium heat.  Bring to a boil and allow the mixture to simmer for about 2 minutes.  Remove from the heat and allow to cool.
5. In a small bowl whisk the egg and milk together.  Set aside.
6. Remove the dough from the refrigerator.  Brush one tray of pastry with the egg wash mixture and spoon 1 teaspoon of the filling onto the center of the rectangles.  Spread jam leaving a 1-inch border around. Using a toothpick, skewer, or chopstick, poke holes in the remaining rectangles to allow the steam to escape during baking.  Place these rectangles ontop of the jam pastries.  Crimp the edges with a fork to seal.  Refrigerate the filled pop tarts for 30 minutes.
7. Preheat oven to 350°F/177°C.  After the chilling period, brush the tops of the pastries with the remaining egg wash.  Bake for 25-30 minutes, until the pop tarts are golden brown.  Allow to cool 5 minutes and then transfer to a wire rack to cool completely.

For the glaze

1. In a medium bowl, whisk together the powdered sugar, cream, and vanilla.  Spoon the glaze over each pop tart.