Preheat the oven to 400°F. Line a baking sheet with parchment paper.
Lightly flour your work surface then unfold the puff pastry. Using a rolling pin, lightly roll out the dough until it is roughly an 11-inch square.
Place the Brie in the center of the square then top it with the jam (optional).
In a small bowl, whisk the egg with 1 tablespoon water.
Fold one of the corners up and over the Brie, then brush it with the egg wash and fold the next corner on top, repeating the egg wash and folding to create a sealed package around the cheese.
Place the Brie on the baking sheet then cut 3 slits on top.
Bake the Brie until the pastry is golden brown and puffed, about 30 minutes. Let it rest for 10 minutes before serving.