Preheat the Oven: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Prepare the Pastry: Roll out the puff pastry on a lightly floured surface into a rectangle (about ⅛ inch thick). Trim the edges if needed.
Cut the Pastry: Cut the pastry into 8 rectangles. Each rectangle should be large enough to enclose a piece of chocolate.
Add Chocolate: Place a small piece of chocolate or a few chocolate chips near one edge of each rectangle.
Shape the Croissants: Roll the pastry tightly around the chocolate, sealing the edges slightly with a pinch or by brushing a little water. Place seam side down on the baking sheet.
Egg Wash: Whisk together the egg and milk, then brush the tops of each croissant with the mixture to give a shiny, golden finish.
Bake: Bake for 20–25 minutes, or until golden brown and puffed up.
Cool and Serve: Let cool for 5 minutes before serving. Dust with powdered sugar or drizzle with melted chocolate if desired.