This refreshing chilled Key lime mousse is a perfect union of cream and foam, tangy lime and soft sweet custard. The method is simple but a little unusual--you make a custard with fresh Key lime juice rather than milk or cream. Key limes, which are about the size of a Ping-Pong ball, are wonderfully fragrant and more acidic than regular limes. They are available all year at well-stocked supermarkets. If you can't find them, by all means use regular limes.