Preheat your oven to 400 degrees. Line a baking sheet with parchment paper.
In a small bowl, mix together the jam and cornstarch. Set aside.
On a floured surface, slightly roll out the pie crusts. Trim the edges of each pie crust with a knife or pizza cutter to create a large rectangle. Slice each pastry into 3 even strips lengthwise, this will create 6 long strips total.
Place a heaping tablespoon of jam onto the bottom half of each strip, leaving about ½ of space around the edges. Brush the bare edges around the jam with the egg wash.
Fold the pastry in half. Use a fork to seal the seams. If any cracks appear in the pastry try to pinch the dough together to create a seal and prevent jam leaking during the baking process. Repeat for the remaining pastry strips.
Poke steam holes with a fork in each pop tart and transfer them to the baking sheet. Brush the top of each pastry with the egg wash.
Bake for 20-22 minutes, or until the pastry is golden brown and cooked through. Allow to cool completely.
In a medium bowl, mix together the powdered sugar and milk. Add additional milk if you’d like a thinner icing glaze. Spread the icing over the pop tarts. Decorate with sprinkles before the icing sets up.