Preheat oven to 400°F (200°C). Roll one puff pastry sheet into a 9x13-inch rectangle and place on a parchment-lined baking sheet.

Stir jam with cornstarch if using, then spread over the pastry, leaving a 1-inch border.

Roll the second pastry sheet to the same size, drape it over the jam, press edges to seal, and crimp with a fork. Chill if pastry is too soft.

Brush with egg wash and prick all over with a fork. Bake 25 to 30 minutes until deep golden. Cool on the sheet at least 15 minutes.

Whisk powdered sugar, milk, and vanilla into a smooth glaze. Drizzle over cooled pop tart, add sprinkles, slice into 8 rectangles, and serve.