This makes a delicious filling for cakes and tarts and can also be used as a spread for biscuits, scones or croissants.
Combine The Juices, Zest, Sugar and butter in a saucepan and bring to a boil over medium heat. Beat the yolks in a bowl until liquid. Beat 1/4 of the boiling liquid into the yolks and return the remaining liquid to a boil over low heat. Beat the yolk mixture into the boiling liquid and continue beating until it thickens slightly, about 2 minutes. Do not allow the curd to boil or it will scramble. Pour into a clean bowl, cover surface with plastic wrap and chill.
