I start by tossing my diced peaches into a saucepan with both sugars, lemon juice, and cinnamon over medium heat. Once the peaches release their juices, I stir in the cornstarch to thicken everything into a jammy, spoonable filling. This usually takes about 8-10 minutes, and I let it cool completely before I touch the pastry, otherwise it turns the dough soggy.
While the filling cools, I roll my pie crust or puff pastry sheets out slightly on a lightly floured surface. I cut each sheet into equal rectangles, usually around 3x4.5 inches, so I get a matching top and bottom for every tart. If you like a bigger bite, you can always go for a giant pop tart puff pastry version by cutting fewer, larger rectangles.
I place about two tablespoons of the cooled peach filling onto one rectangle, leaving a small border around the edges. I brush that border with egg wash, then lay a second rectangle on top and press the edges together firmly. This step matters most for keeping the filling inside while baking.
Using a fork, I crimp all four edges to seal them completely, then poke a few small holes on top so steam can escape. I always chill my assembled tarts for at least 30 minutes, since cold pastry holds its shape and bakes up flakier than dough straight from the counter.
I brush the tops with the remaining egg wash and bake at 375°F for 22-25 minutes, until the pastry turns deep golden brown. I rotate the tray halfway through baking so every tart browns evenly, and I let them cool fully on a wire rack before glazing.
Once the tarts have cooled, I whisk powdered sugar, milk, and vanilla into a smooth glaze and drizzle it generously over the top. I like to let the glaze set for about ten minutes before serving, so it firms up into that classic pop tart shine.