People rarely make Sachertorte (Austrian mirror glaze cake) at home. It is more likely to be seen in the window displays of beautiful patisseries around the world. The method differs from the usual one for making a cake, as you mix the egg whites in separately to retain a super-light sponge. It is easy to do, but it takes a bit of juggling. The best bit, of course, is the chocolate glaze, dripping quickly down the sides of this very special cake.