Preheat the oven to 350°F (180°C). Line a large baking sheet with parchment paper or a silicone baking mat.
Unfold the puff pastry onto the prepared baking sheet. Place the chocolate bar down the center of the pastry. If needed, gently roll the pastry out slightly so the chocolate fits comfortably.
Using a sharp knife or pizza cutter, cut diagonal strips down both sides of the chocolate bar, about 1 inch wide.
Fold the strips over the chocolate, alternating sides to form a braid. Press the ends gently to help seal. If the pastry feels soft, refrigerate the baking sheet for 10 minutes.
Beat the egg and brush it all over the top of the braid.
Bake for 10 to 15 minutes, or until puffed and deeply golden. Start checking at 10 minutes, as puff pastry can brown quickly.
Cool for 5 to 10 minutes. Dust with powdered sugar, slice, and serve warm.