1. 1. Prepare Your Workspace and Preheat Oven: Start by preheating your oven to 375 F (190 C). Line a baking sheet with parchment paper to prevent sticking and make cleanup easier. Lightly dust a clean work surface with a little all-purpose flour.

2. 2. Unroll and Combine Dough: Carefully unroll the crescent roll dough onto your floured surface. Do not separate the individual triangles. Gently press the seams together to form one large rectangle. Spread the sourdough discard evenly over the entire surface of the dough.

3. 3. Fold and Roll: Fold the dough in half lengthwise, bringing one long edge over to meet the other. Gently press down. Now, use a rolling pin to roll the dough out into a rectangle, approximately 12×8 inches and about 1/8-inch thick. The sourdough discard will be incorporated into the dough as you roll.

4. 4. Cut and Fill: Using a sharp knife or a pizza cutter, cut the dough into 8 equal rectangles. Spoon about 1-2 tablespoons of your chosen fruit preserves onto the center of four of the dough rectangles, leaving a small border around the edges.

5. 5. Assemble the Pop Tarts: Place the remaining four dough rectangles over the top of the fruit-filled ones. Gently press down around the edges to seal the filling inside. Use the tines of a fork to crimp the edges all around each pop tart, ensuring a tight seal to prevent the filling from leaking out during baking. Prick the top of each pop tart a few times with the fork to allow steam to escape.

6. 6. Bake to Golden Perfection: Carefully transfer the assembled pop tarts to your prepared baking sheet. Bake for 12-15 minutes, or until the pastry is golden brown and puffed up. Keep an eye on them to prevent over-browning.

7. 7. Prepare and Glaze: While the pop tarts are baking, whisk together the powdered sugar, milk, and vanilla extract in a small bowl until smooth and lump-free. Once the pop tarts are out of the oven, let them cool on the baking sheet for a few minutes before transferring them to a wire rack. Drizzle generously with the prepared glaze while they are still warm, and add sprinkles if desired.