1. 1. Prepare the Dough: In a large bowl, combine the gluten-free flour, sugar, and salt. Add the cold, cubed butter and cut it into the flour mixture using a pastry blender or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

2. 2. Add Wet Ingredients: Stir in the gluten-free sourdough discard. 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.

3. 3. Chill the Dough: Form the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes. This chilling period is crucial for the gluten-free dough to hydrate and become easier to handle.

4. 4. Assemble the Pop Tarts: On a lightly floured surface (using gluten-free flour), roll out the chilled dough to about 1/8-inch thickness. Using a sharp knife or pizza cutter, cut the dough into rectangles, approximately 3×4 inches. You should get about 12-16 rectangles.

5. 5. Fill and Seal: Place about 1-2 teaspoons of your chosen fruit preserves in the center of half of the dough rectangles, leaving a small border around the edges. Lightly brush the edges of the filled rectangles with the beaten egg wash. Top with the remaining dough rectangles. Use a fork to crimp the edges firmly, sealing the pop tarts. Prick the top of each pop tart a few times with the fork to allow steam to escape during baking.

6. 6. Bake the Pop Tarts: Preheat your oven to 375 F (190 C). Line a baking sheet with parchment paper. Carefully transfer the assembled pop tarts to the prepared baking sheet. Brush the tops of the pop tarts with the remaining egg wash. Bake for 15-18 minutes, or until the crust is golden brown and flaky.

7. 7. Prepare the Glaze and Finish: While the pop tarts are cooling slightly, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth. Once the pop tarts have cooled for about 10-15 minutes, drizzle the glaze over them. If desired, add sprinkles immediately before the glaze sets. Serve warm or at room temperature.