01 - In a large mixing 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 with some pea-sized pieces remaining. Drizzle in ice water one tablespoon at a time, tossing gently with a fork, until the dough just holds together when squeezed. 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 gentle 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 noticeably. Remove from heat and let cool completely.
03 - Preheat the oven to 375°F. Line a large baking sheet with parchment paper and set aside.
04 - On a lightly floured work surface, roll out each chilled dough disc to approximately ⅛-inch thickness. Using a knife or pastry cutter, trim and cut each disc into 8 rectangles, roughly 3 × 4 inches each, for a total of 16 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 ¼-inch border around the edges. Lightly brush the exposed borders with egg wash. Lay the remaining 8 rectangles on top and press the edges firmly to seal. Crimp the edges with the tines of a fork for a decorative seal.
06 - Brush the tops of each assembled pop tart evenly with the remaining egg wash. Use a fork to poke 2 to 3 small steam holes in the top crust of each pastry.
07 - Bake on the center rack for 18 to 22 minutes, until the tops are deeply golden brown and the edges are lightly crisped. Transfer to a wire rack and allow to cool completely before glazing.
08 - In a small bowl, whisk together the powdered sugar, milk, vanilla extract, and peach jam if using, until a smooth pourable glaze forms. Adjust consistency with additional milk or sugar as needed. Drizzle generously over the cooled pop tarts. Let the glaze set for 15 to 20 minutes before serving.