01 - In a large bowl, whisk together the flour, sugar, and salt. Cut in the cold cubed butter using a pastry cutter or two knives until the mixture resembles coarse crumbs.
02 - Gradually add ice water, stirring until the dough just comes together. Divide into two equal disks, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes.
03 - Combine the diced peaches, sugar, lemon juice, and cornstarch in a saucepan. Cook over medium heat, stirring frequently, for 5 to 7 minutes until the mixture thickens. Remove from heat and let cool completely.
04 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
05 - On a lightly floured surface, roll each chilled dough disk to approximately 1/8-inch thickness. Cut into 3x4-inch 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 small border around the edges.
07 - Brush the edges of each filled rectangle with egg wash. Place a second dough rectangle on top of each and press the edges firmly with a fork to seal.
08 - Brush the tops of each assembled pastry with egg wash. Prick the tops several times with a fork to create steam vents.
09 - Bake for 20 to 25 minutes until the pastries are golden brown. Transfer to a wire rack and let cool completely.
10 - Whisk together the powdered sugar, milk, and vanilla extract until smooth. Drizzle or spread the glaze over the cooled pop tarts. Top with colored sprinkles if desired.