Defrost pie crust according to package directions. Note that if you keep it in the refrigerator instead of the freezer, it will still need to sit out 10-15 minutes before unrolling.
Preheat oven to 400 F. Line a baking sheet with parchment paper.
Mix 1 cup of cooled apple jam with the cornstarch.
Roll out the pie crust, into a square if you can (or cut off the edges of the circle).
Cut into strips about 3" wide.
Spoon the jam onto one half of each strip, leaving the edges uncovered.
Brush the edges with egg, and then fold the strip in half so that the empty side of the strip folds over top of the jam. Seal the edges with your fingers, and then press edges with a fork.
Brush the top of each poptart with egg and bake for 15-18 minutes.