01 - In a large bowl, whisk together the flour, sugar, and salt. Cut in the cold cubed butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing gently until the dough just comes together. Divide the dough in half, shape each portion into a flat disc, wrap tightly in plastic, and refrigerate for at least 30 minutes.
02 - While the dough chills, combine the diced peaches, granulated sugar, lemon juice, and cinnamon in a small saucepan. Bring to a gentle simmer over medium heat, stirring frequently. Dissolve the cornstarch in 2 teaspoons of water and stir into the peach mixture. Continue cooking for 2 to 3 minutes until thickened. Remove from heat and let cool completely.
03 - Preheat the oven to 375°F. Line a baking sheet with parchment paper.
04 - On a lightly floured surface, roll out each chilled dough disc to 1/8-inch thickness. Cut each disc into 8 rectangles, approximately 3 by 4 inches, yielding 16 total rectangles.
05 - Place 8 dough rectangles on the prepared baking sheet. Spoon 1 to 2 tablespoons of cooled peach filling onto the center of each rectangle, leaving a 1/4-inch border. Lightly brush the exposed edges with egg wash. Lay the remaining 8 rectangles on top and press the edges firmly to seal. Crimp the borders with a fork to ensure a tight seal.
06 - Brush the tops of each assembled pastry with the remaining egg wash. Use a fork to poke 2 to 3 small holes in the top of each pastry to allow steam to escape during baking.
07 - Bake for 18 to 22 minutes, or until the pastry tops are golden brown. Transfer to a wire rack and allow to cool completely.
08 - In a small bowl, whisk together the powdered sugar, milk, vanilla extract, and peach jam if using, until a smooth pourable glaze forms. Drizzle generously over the cooled pastries. Allow the glaze to set for 10 to 15 minutes before serving.