To make the egg wash, put the egg, milk and salt in a bowl and whisk with a fork until well combined. Set aside until needed.
Take the rested dough from the refrigerator and roll it out into a rectangle, about 25 x 100 cm (10 x 39½ inches) and about 5–8 mm (¼-⅜ 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 rolled dough into 18 triangles with a base that is 9 cm (3½ inches) wide and two sides that are 21 cm (8¼ inches) high — you may wish to prepare a template by drawing the dimensions on a piece of cardboard and cutting out a triangle to use as a guide.
Stack the triangles on a tray lined with baking paper, cover lightly with a clean tea towel (dish towel) and place in the refrigerator for about 10 minutes. Remove from the refrigerator and, working with one triangle at a time, gently stretch it out slightly. Starting from the base end roll up towards the tip, pressing the tip to secure (see pictures).
Preheat the oven to 240°C (475°F/Gas 8). Place the rolled croissants back onto the lined trays at well-spaced intervals. Cover loosely with a damp tea 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 croissant 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.