1. 1. Prepare Your Workspace and Preheat Oven: Start by preheating your oven to 400 F (200 C). Line two baking sheets with parchment paper or silicone baking mats to prevent sticking and make cleanup easier. This ensures your pop tarts will bake evenly and release effortlessly.

2. 2. Unroll and Cut Puff Pastry: Carefully unroll one sheet of thawed puff pastry on a lightly floured surface. Using a sharp knife or a pizza cutter, cut the pastry into 6 equal rectangles. Repeat with the second sheet of puff pastry, so you have a total of 12 rectangles. These will form the tops and bottoms of your pop tarts.

3. 3. Spread Sourdough Discard: On 6 of the pastry rectangles, spread a thin, even layer of sourdough discard, leaving a small border around the edges. This layer is subtle but adds a unique depth of flavor and helps create a slight barrier for the jam.

4. 4. Add the Jam Filling: Spoon about 1-2 tablespoons of your chosen fruit jam onto the center of each discard-covered rectangle. Be careful not to overfill, as this can cause the jam to leak out during baking. Leave a clear border around the edges for sealing.

5. 5. Assemble and Seal the Pop Tarts: Place the remaining 6 plain pastry rectangles directly on top of the jam-filled ones. Gently press down around the edges to seal. Use the tines of a fork to crimp all four sides of each pop tart, creating a decorative edge and ensuring a tight seal to prevent the filling from escaping.

6. 6. Apply Egg Wash and Bake: In a small bowl, whisk the egg with a tablespoon of water to create an egg wash. Lightly brush the tops of each assembled pop tart with the egg wash. This will give them a beautiful golden-brown color and a slight sheen. Carefully transfer the pop tarts to your prepared baking sheets.

7. 7. Bake Until Golden: Bake for 15-20 minutes, or until the pastry is puffed, golden brown, and cooked through. Keep an eye on them, as oven temperatures can vary. Once baked, remove the pop tarts from the oven and let them cool on the baking sheets for a few minutes before transferring them to a wire rack to cool completely.

8. 8. Prepare and Drizzle Glaze: While the pop tarts are cooling, prepare the glaze. In a small bowl, whisk together the powdered sugar, 1 tablespoon of milk or water, and vanilla extract (if using) until smooth. Add more milk or water a teaspoon at a time until you reach your desired drizzling consistency. Once the pop tarts are cool, drizzle the glaze over them and add sprinkles, if desired, before the glaze sets.