Preheat the oven to 375 degrees F and line a large rimmed baking sheet with parchment paper.
Unroll one sheet of pie crust on a lightly floured surface and use a rolling pin to shape it into a rough 10 by 13 inch rectangle, patching edges with scraps if needed.
Transfer the crust to the prepared baking sheet and spread the cherry jam over the center, leaving a 1 inch border on all sides; mix in cornstarch first if your jam is very runny.
Brush the exposed border with the beaten egg wash so the top crust will seal firmly.
Roll out the second sheet of pie crust to a similar size and carefully place it over the jam; press the edges together with your fingers, then crimp all around with a fork.
Use a fork to poke small vents across the top so steam can escape, then brush the entire surface with more egg wash.
Bake the giant pop tart for 25 to 30 minutes, or until the crust is evenly golden and smells toasty; cool on the pan until just slightly warm.
While it cools, whisk together the powdered sugar, milk, vanilla, and salt until smooth and pourable but not too thin.
Pour or spoon the icing over the warm pop tart, gently spreading it toward the corners while leaving a small border.
Immediately sprinkle the top with rainbow sprinkles and let the icing set for 10 to 15 minutes before slicing and serving.