To make the filling. In a medium bowl add the finely chopped peaches, brown sugar and cinnamon. Mix well and set aside.
In a food processor, combine the all purpose flour, cold butter, sugar and salt. Pulse until the mix clumps together to forms pea-size balls. Add ½ cup of ice cold water, 1 tablespoon at a time, until the dough comes together and forms a ball. If need be, add more cold water, 1 tablespoon at a time, if the dough is too dry.
Turn the dough out onto a floured work surface. Roll the dough out into a ⅛-inch thickness and cut the dough into 24 rectangles, about 4 x 3 inches big. In the middle of 12 rectangles, spoon 1 tablespoon of the brown sugar peach filling. Beat the egg and add a splash of water to the beaten egg. Brush the edges of the pop tarts with the egg/water mixture (this will help the dough seal), and place the remaining half of the dough over the rectangles with filling. Seal the pop tarts by pressing a fork along the edges of each pop tart. Repeat. You should have around 12 assembled pop tarts when finished.
When the pop tarts are all assembled, carefully transfer the pop tarts to a parchment-lined baking sheet. With a sharp knife, make a few slits in the tops of the pop tarts. Place the baking sheet in the freezer for 20 minutes to chill.
While the pop tarts are chilling, preheat the oven to 400 degrees F. Bake the pop tarts for 15-20 minutes, or until golden brown. While the pop tarts are cooling, start on the brown butter glaze.
In a small saucepan over medium high heat add the butter. Whisk for 5 minutes, or until the butter browns and has a 'nutty' aroma. Remove from the heat and allow to cool for 5 minutes. Next, whisk in the powder sugar, vanilla, and a pinch of salt. If the glaze needs to be thinner, you can add 1 tablespoon of water at a time until you reach the desired consistency. Once the pop tarts are completely cool- drizzle the glaze over the top, serve and enjoy!