Beat the egg white until it is frothy but not stiff. Sieve the icing sugar and mix it gradually into the egg white until the mixture is a firm paste. Add peppermint to taste (oil of peppermint gives the best flavour, although peppermint essence can be used). Knead the mixture until it is smooth, and then roll it out on a board dusted with icing sugar. Cut into small circles. If preferred, the mixture can be rolled into small balls with the hands and then flattened with a fork. Leave overnight to become firm. When firm, these peppermint creams can be dipped into melted plain chocolate if liked.