Step 1: Make the Dough

1. In a large bowl, whisk together the flour, sugar, and salt.
2. Cut in the cold butter using a pastry cutter or food processor until the mixture resembles coarse crumbs.
3. Gradually add the ice water, mixing until the dough comes together. Do not overwork the dough.
4. Divide the dough in half, shape into discs, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Step 2: Prepare the Filling

1. If using jam, mix it with cornstarch in a small bowl to prevent it from leaking out during baking.

Step 3: Roll and Cut the Dough

1. On a floured surface, roll out one dough disc into a ⅛-inch thick rectangle.
2. Cut into 3×4-inch rectangles using a knife or pizza cutter. Repeat with the second dough disc.
3. Place half of the rectangles on a parchment-lined baking sheet.

Step 4: Fill and Seal the Poptarts

1. Spoon about 1 tablespoon of filling onto the center of each bottom rectangle.
2. Brush the edges with the egg wash, then place a second rectangle on top.
3. Use a fork to crimp the edges, sealing the poptarts.
4. Poke a few holes on the top with a fork to allow steam to escape.

Step 5: Bake

1. Preheat oven to 375°F (190°C).
2. Brush the tops of the poptarts with the remaining egg wash.
3. Bake for 20-25 minutes or until golden brown.
4. Let them cool completely before glazing.

Step 6: Prepare the Glaze

1. In a small bowl, mix powdered sugar, milk, and vanilla extract until smooth.
2. Drizzle over the cooled poptarts and add sprinkles if desired.
3. Let the glaze set before serving.