Preheat the oven to 425 degrees F. Unfurl the sheet of pastry and then cut it into 6 squares.
Cut each square diagonally to give 2 triangles (they will appear quite small). Put the triangle with the wider part facing you and the point away from you.
Break off small pieces of chocolate (approx.1cm/half inch) to place about 2cm/3/4-inch up from the wide end nearest you.
Then carefully roll from that chocolate loaded end towards the point of the triangle.
You should now have something resembling a straight croissant, seal it slightly with your fingertips and curl it around into a crescent.
Place the chocolate croissants on a lined baking tray and paint with the beaten egg. Bake for 15 minutes until golden and puffy and exuberantly, if miniaturely, croissant-like.