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 just until the dough 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, sugar, lemon juice, and cinnamon in a small saucepan. Bring to a simmer over medium heat, stirring frequently. Dissolve the cornstarch in 2 teaspoons of water and stir it into the peach mixture. Continue cooking for 2 to 3 minutes until the filling thickens. 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 each dough disc out to 1/8-inch thickness. Cut into 16 rectangles, approximately 3 by 4 inches each.
05 - Place 8 dough rectangles on the prepared baking sheet. Spoon 1 to 2 tablespoons of cooled peach filling onto each rectangle, leaving a small border around the edges. Brush the borders lightly with egg wash. Lay the remaining 8 rectangles on top and press the edges firmly to seal. Crimp the edges with a fork to secure.
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 one to allow steam to escape during baking.
07 - Bake for 18 to 22 minutes until the tops are golden brown. Transfer to a wire rack and let cool completely.
08 - In a small bowl, whisk together the powdered sugar, milk, vanilla extract, and peach jam if using, until smooth and pourable. Drizzle the glaze over the cooled pastries. Allow the glaze to set for 10 to 15 minutes before serving.