Preheat oven to 400ºF. Line a baking sheet with parchment paper and set aside. In a small bowl, mix ½ cup jam with cornstarch. Crack the egg into a second small bowl and whisk. Flour a large work surface and lay both pie crusts out on it. Roll the circles to 11 inches across. Use a knife or pizza cutter to cut the edges off, about 1 inch off all sides, to create two 9-inch squares. Then cut each square into three 3-inch stripes, or 6 total.Spoon 1-2 tablespoons of jam on one end of each strip. Brush a rectangle of egg around the jam, to glue the sides together. Fold the dry end over the end with jam.Use a fork to crimp the edges together on all sides. Repeat with remaining pie crust strips. Move the pop tarts to the prepared baking sheet. Bake for 15–18 minutes, until golden. Cool.After the pop tarts have cooled for several minutes, whisk powdered sugar, 1 tablespoon jam, and 1-2 tablespoons cream together to create an icing glaze. Spoon over the top of each pop tart and immediately sprinkle with sugar sprinkles. Let the icing dry, either on the baking sheets of on a dry rack.Serve warm or at room temperature.