Take the rested dough from the refrigerator and roll it out into a rectangle, about 35 x 100 cm (14 x 39½ inches) and about 5 mm (¼ inch) thick. Rolling the dough may take quite a lot of energy as it may keep springing back to its original size. If this happens, rest the dough in the refrigerator for about 10 minutes then resume rolling. This resting process may need to be done a number of times. If the dough is becoming too large to fit in the refrigerator simply fold it over and place on a tray before placing it in.
Cut the dough into 24 rectangles, about 8 x 16 cm (3¼ x 6¼ inch) each. Place the rectangles on trays lined with baking paper and place in the refrigerator for 10 minutes. Remove from the refrigerator. Using 10 g (¼ oz) of the chocolate, place a line of the chopped pieces at the narrow end closest to you and fold the dough over to enclose. Using another 10 g (¼ oz) chocolate, place another line to sit beside the first and continue rolling the dough to form a log (see pictures). Repeat with the remaining pastry pieces and chocolate.
Preheat the oven to 240°C (475°F/Gas 8). Place the pan au chocolat back onto the lined trays at well-spaced intervals, seam side down. Cover loosely with a damp tea towel (dish towel). Set aside in a warm room (about 25°C–27°C/77°F–81°F) for 1½–2 hours, or until almost doubled in size. Spray the tea towel with water occasionally if it becomes dry.
Remove the tea towel, brush the top of each pan au chocolat lightly with egg wash and place in the oven. Reduce the oven temperature to 190°C (375°F/Gas 5) and bake for about 15 minutes, or until a deep golden colour. Cool slightly on the trays before serving.