Combine flour, sugar, and salt in a large mixing bowl.

Add the cold butter cubes and use a pastry cutter or your fingertips to work the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter still visible.

Gradually add ice water one tablespoon at a time, mixing gently until the dough just comes together.

Divide the dough in half, shape each portion into a flat disk, wrap in plastic wrap, and refrigerate for at least 30 minutes.

While the dough chills, stir together the jam and cornstarch in a small bowl until well combined. The cornstarch prevents the filling from becoming too runny during baking.

Preheat your oven to 375°F and line a large baking sheet with parchment paper.

On a lightly floured surface, roll one disk of dough into a large rectangle, about 10x14 inches and roughly ¼ inch thick. Transfer carefully to the prepared baking sheet.

Roll out the second disk to the same size and set aside.

Spread the jam mixture over the dough rectangle on the baking sheet, leaving a 1-inch border around all edges.

Brush the exposed border with beaten egg to act as glue to seal the layers together.

Carefully place the second rectangle of dough on top and press the edges firmly with a fork to seal, creating a classic crimped pattern around the perimeter.

Poke several holes across the top with a fork to allow steam to escape, then brush the entire surface with more egg wash.

Bake for 23-27 minutes until the pastry turns deep golden brown and the edges look crisp.

Let it cool on the baking sheet for 10 minutes before adding the icing.

Whisk together powdered sugar, milk, and vanilla until smooth and pourable but not too thin.

Drizzle or spread the icing generously over the cooled pop tart, then immediately add sprinkles before the icing sets.

Slice into squares and serve warm.