1. 1. Prepare the Dough: In a large mixing bowl, combine the gluten-free flour, granulated sugar, and salt. Add the cold, cubed butter and use a pastry blender or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining. Stir in the gluten-free sourdough discard until just combined. Gradually add the ice water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix; the dough should be shaggy but hold together when pressed.

2. 2. Chill and Roll the Dough: Form the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes. This chilling step helps the butter firm up and makes the dough easier to handle. Once chilled, lightly flour a clean surface with gluten-free flour. Roll out the dough into a rectangle, about 1/8 to 1/4 inch thick. Aim for a size that allows you to cut out approximately 12 rectangles, each about 3×4 inches. Use a sharp knife or pizza cutter for clean edges.

3. 3. Fill the Pop Tarts: Lay half of the dough rectangles on a parchment-lined baking sheet. Spoon about 1-2 tablespoons of your chosen fruit preserves onto the center of each dough rectangle, leaving a small border around the edges. Be careful not to overfill, as this can cause the jam to leak out during baking.

4. 4. Assemble and Seal: Place the remaining dough rectangles on top of the jam-filled ones. Gently press down around the edges of each pop tart to seal them. Use the tines of a fork to crimp the edges firmly, creating a decorative seal that also helps prevent the filling from escaping. Prick the top of each pop tart 2-3 times with a fork to allow steam to escape during baking.

5. 5. Egg Wash and Bake: In a small bowl, lightly beat the egg. Brush the tops of all the assembled pop tarts with the egg wash. This will give them a beautiful golden-brown color and a slight sheen. Preheat your oven to 375 F (190 C). Bake the pop tarts for 15-18 minutes, or until they are golden brown and puffed up slightly.

6. 6. Cool and Glaze: Once baked, remove the pop tarts from the oven and let them cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. While they are cooling, prepare the glaze: In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth and drizzleable. Once the pop tarts are cool, drizzle the glaze over them. If desired, add sprinkles immediately before the glaze sets. Allow the glaze to harden slightly before serving.