Preheat Oven and Prep Baking Sheet:
Move the oven racks to the upper-middle and lower-middle positions. Preheat the oven to 400°F (200°C).
Line two baking sheets with parchment paper.
Prepare Puff Pastry and Chocolate:
Remove the puff pastry sheets from the freezer and let them thaw for at least 15 minutes.
Unfold one puff pastry sheet on a lightly floured surface. Cut it into 8 equal pieces.
Break each chocolate bar into 4 sections, then place one section of chocolate in the center of each puff pastry piece.
Make Egg Wash and Assemble:
In a small bowl, whisk together the egg and 1 tablespoon of water to make the egg wash.
Fold the sides of each puff pastry piece over the chocolate, then wrap the top over the sides and press to seal the pastry at the bottom.
For extra sealing, use a bit of egg wash where the pastry overlaps.
Place the assembled croissants on the prepared baking sheets and brush the tops and sides with egg wash.
Bake and Cool:
Bake for 14-15 minutes, or until the pastry is golden brown, rotating the baking sheets halfway through.
Transfer the baking sheets to wire racks and let the croissants cool for 10 minutes.
Finish:
Dust the croissants with powdered sugar and serve warm.