Fold butter inside dough.
Roll out sheet of dough and divide into long triangles.
Let rise for a couple of hours in a warm place. Keep damp.
Brush with beaten egg and cook around 20mins -- start at 200c then drop to 160c.
Eat! Croissants you haven't cooked can be frozen for up to a week--put them in before rising, then allow a few hours to rise and defrost when you take them out.