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 with pea-sized pieces remaining. Drizzle in ice water one tablespoon at a time, tossing gently after each addition, 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 - In a small saucepan, combine the diced peaches, granulated sugar, lemon juice, and ground cinnamon. Bring to a gentle simmer over medium heat, stirring frequently to prevent sticking. Dissolve the cornstarch in 2 teaspoons of cold water, then stir the slurry into the peach mixture. Continue cooking for 2 to 3 minutes until the filling thickens and turns glossy. Remove from heat and let cool completely before using.
03 - Preheat the oven to 375°F (190°C). Line a large baking sheet with parchment paper and set aside.
04 - On a lightly floured surface, roll out each chilled dough disc to approximately 1/8-inch thickness. Using a knife or pastry cutter, trim and cut each disc into 8 rectangles measuring about 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 around the edges. Lightly brush the exposed borders with beaten egg wash. Lay the remaining 8 dough rectangles on top and press the edges firmly to seal. Crimp the sealed edges with the tines of a fork for a decorative finish.
06 - Brush the tops of each assembled pastry with the remaining egg wash for a golden finish. Use a fork or toothpick to poke 2 to 3 small steam vents in the top of each pop tart.
07 - Bake on the center rack for 18 to 22 minutes, rotating the pan halfway through, until the pastry is deeply golden brown. Transfer to a wire cooling 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 smooth and pourable. Adjust consistency with additional milk or sugar as needed. Drizzle the glaze over the cooled pop tarts and let it set for 15 to 20 minutes before serving.