01 - In a large mixing bowl, whisk together the flour, sugar, and salt. Cut the cold cubed butter into the dry ingredients using a pastry cutter or two knives until the mixture resembles coarse crumbs with pea-sized butter pieces throughout.
02 - Gradually drizzle in the ice water, stirring gently until the dough just begins to come together. Divide into two equal disks, wrap each tightly in plastic wrap, and refrigerate for at least 30 minutes.
03 - Combine the diced peaches, sugar, lemon juice, and cornstarch in a medium saucepan. Cook over medium heat, stirring frequently, for 5 to 7 minutes until the mixture thickens and becomes glossy. Remove from heat and let cool completely.
04 - Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper and set aside.
05 - On a lightly floured surface, roll each chilled dough disk to approximately 1/8-inch (3 mm) thickness. Cut into 3x4-inch (8x10 cm) rectangles, yielding 16 total rectangles.
06 - Place 8 dough rectangles on the prepared baking sheet. Spoon about 1 rounded tablespoon of cooled peach filling onto the center of each rectangle, leaving a 1/2-inch border around the edges.
07 - Brush the exposed borders with egg wash. Lay a second dough rectangle over each filled piece and press the edges firmly with a fork to seal. Brush the tops with remaining egg wash and prick the surface with a fork to create steam vents.
08 - Bake for 20 to 25 minutes until the pastry is golden brown and cooked through. Transfer to a wire rack and cool completely before glazing.
09 - Whisk together the powdered sugar, milk, and vanilla extract until smooth and pourable. Drizzle or spread the glaze over the cooled pop tarts and top with sprinkles if desired. Allow the glaze to set before serving.