Whisk together flour and salt, then cut in very cold butter until coarse crumbs form. Gradually add ice water and mix until the dough just comes together, being careful not to overmix.

Divide the dough into two flattened discs, wrap them in plastic, and refrigerate for at least 30 minutes.

On a lightly floured surface, roll out one dough disc to 1/8-inch thick and trim to a 9x12-inch rectangle. Cut into 6 smaller 3x4-inch rectangles.

Repeat with the second dough disc to create a total of 12 dough rectangles.

Place 6 dough rectangles on a parchment-lined baking sheet and spoon 1-2 tablespoons of fruit filling onto the center of each, leaving a ½-inch border. If jam is runny, mix with cornstarch first.

Brush the edges of the dough with whisked egg and top with the remaining 6 dough rectangles.

Gently press to seal the edges and then crimp all around the perimeter of each tart with a fork to prevent leaks. Prick the top of each tart a few times for steam vents.

Refrigerate the baking sheet for 15 minutes while preheating the oven to 375°F (190°C).

Bake for 15-18 minutes until golden brown and slightly puffy, then cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.

Whisk powdered sugar, milk, and vanilla (if using) to make a smooth glaze. Drizzle over cooled Pop Tarts, add sprinkles if desired, and let the glaze set before serving.