Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Prepare the puff pastry. Place the thawed pastry on a lightly floured surface and gently roll it if needed to even out the sheet.
Cut the pastry. Cut the sheet into 12 similar rectangles. Six will be the bottoms and six will become the tops of the pop tarts.
Add the filling. Place about 1 tablespoon of jam in the center of six pastry rectangles. Leave a clear border around the edges.
Cover the filling. Place another pastry rectangle over each filled piece.
Seal the edges. Press around the edges with your fingers first, then crimp them firmly with a fork.
Prepare the egg wash. Beat the egg with 1 tablespoon of water in a small bowl.
Brush the tops. Lightly brush each pastry with the egg wash.
Add small vents. Make two or three small cuts in the top of each pastry so steam can escape while baking.
Bake the puff pastry pop tarts. Place the tray in the oven and bake for about 15–20 minutes, until the pastry is puffed and golden brown.
Cool briefly. Let the pastries sit on the baking sheet for several minutes before moving them to a cooling rack.
Make the glaze. Mix powdered sugar with a small amount of milk or water until smooth and thick enough to drizzle.
Finish the pop tarts. Drizzle the glaze over the cooled pastries and add sprinkles if desired.