Mix in a saucepan the barley, onion, cinnamon, mace and water. When the mixture boils remove from the center of the fire and allow to simmer slowly for three hours or more. Pass through a fine sieve and return to saucepan. Mix in the butter and boiling milk, season with pepper and salt. Beat the egg yolk in the teacupful of milk, mix in the soup but do not allow to boil after egg is added. Serve with croutons.