In a large bowl, stir the flour and salt, then cut in cold butter with a fork or pastry blender. The mixture should be sandy with some pea-sized crumbles. Add cold water and mix until the dough forms. Pat into two disk shapes, wrap with plastic wrap and refrigerate for 2-3 hours.

Preheat the oven to 350 degrees Fahrenheit and line a large baking sheet with parchment paper. On a lightly floured work surface, roll one of the dough discs into a large rectangle about ⅛ inch thick. Trim the edges of the rectangle with a sharp knife or a pastry cutter as needed. Cut the dough into eight equal-sized rectangles.

Place four of the rectangles on the prepared baking sheet. Spoon a heaping tablespoon of jam or preserves into the center of the four rectangles, leaving room around the edges to seal the tarts. Position the remaining four rectangles carefully over the top of the jam, using a fork to crimp the edges together. Trim the edges if desired for a more uniform appearance. Repeat with the remaining dough until you have eight pop tarts assembled.

Whisk the egg with a splash of water, then use a pastry brush to lightly apply the egg wash to the top of each pop tart. Bake for 22-30 minutes or until the dough is golden brown and cooked through.

Remove from the oven and let cool completely. While the pop tarts cool, add the powdered sugar and milk to a small bowl, stirring to form the icing.

Once cooled, top each pop tart with icing. Add sprinkles if you'd like. Let the icing set and enjoy!