01 - In a large mixing 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 with pea-sized pieces remaining.
02 - Gradually drizzle in the ice water, stirring gently 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 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. Line a baking sheet with parchment paper and set aside.
05 - On a lightly floured surface, roll each chilled dough disk to approximately ⅛-inch thickness. Cut into 3×4-inch rectangles, yielding 16 total rectangles.
06 - Place 8 dough rectangles on the prepared baking sheet. Spoon a rounded tablespoon of cooled peach filling onto the center of each rectangle, leaving a ½-inch border around the edges.
07 - Lightly 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 crimp and seal.
08 - Brush the tops of each assembled tart with the remaining egg wash. Prick the surface 3 to 4 times with a fork to create steam vents during baking.
09 - Bake for 20 to 25 minutes until the pastry is golden brown and flaky. Transfer to a wire rack and allow to cool completely.
10 - Whisk together the powdered sugar, milk, and vanilla extract until smooth and drizzly. Spread or drizzle over the cooled tarts. Top with colored sprinkles if desired. Allow glaze to set before serving.