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 into a flat disc, wrap tightly in plastic, and refrigerate for at least 30 minutes.
02 - While the pastry 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, then stir it into the peach mixture. Continue cooking for 2–3 minutes until thickened. Remove from heat and let the filling cool completely.
03 - Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
04 - On a lightly floured surface, roll out each dough disc to 1/8-inch thickness. Cut into 16 rectangles, approximately 3 x 4 inches each.
05 - Place 8 rectangles on the prepared baking sheet. Spoon 1–2 tablespoons of cooled peach filling onto each rectangle, leaving a small border around the edges. Brush the borders 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 tart with egg wash. Use a fork to poke 2–3 small holes in the top crust to allow steam to escape during baking.
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 drizzleable. Spoon or drizzle the glaze over the cooled pop tarts. Allow the glaze to set before serving.