Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper to keep clean-up easy.
Lightly flour your work surface and gently roll out the puff pastry sheets. Cut each sheet into 4 equal rectangles, aiming for even sizes so they bake up nicely.
Spoon about 1 tablespoon of your chosen jam into the center of half of the rectangles. Make sure to leave a small border around the edges so the jam doesn't ooze out during baking.
Brush the edges of the pastry with the beaten egg. This acts like glue! Place the other pastry rectangles on top, then use a fork to press and seal the edges together firmly.
Transfer the pop tarts onto the prepared baking sheet. Brush the tops with the remaining egg wash to help them bake up golden and glossy. Then poke a few small holes in the tops with a fork — this lets steam escape and keeps the pastry from puffing up too much.
Pop them into the oven and bake for 12-15 minutes or until they're beautifully golden brown and puffed up.
Let the pop tarts cool slightly on a wire rack. Meanwhile, whisk together the powdered sugar, vanilla extract (if using), and water until smooth. Once the pop tarts have cooled, drizzle the frosting over the tops and immediately sprinkle with sprinkles if you’re using them.
Allow the frosting to set before diving in. Then enjoy every flaky, sweet bite!