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. Add ice water one 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 - In a small saucepan, combine the diced peaches, sugar, lemon juice, and cinnamon. 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 2–3 minutes until thickened. Remove from heat and cool completely before using.
03 - Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper and set aside.
04 - On a lightly floured surface, roll each dough disc out to 1/8-inch thickness. Cut into 16 rectangles approximately 3 x 4 inches each.
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. Brush the edges lightly with egg wash. Top each with a remaining dough rectangle and press the edges firmly to seal. Crimp the edges with a fork to secure.
06 - Brush the tops of each assembled pastry with egg wash. Use a fork to poke 2–3 small steam holes in the top of each pop tart.
07 - Bake for 18–22 minutes, or until the pastry is 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 drizzly. Spoon or drizzle the glaze over the cooled pop tarts. Allow the glaze to set for 10–15 minutes before serving.