In a large bowl, cream butter and sugar together with an electric mixer until a paste is formed. Add 1 egg at a time while mixing and then add yolks one at a time. Beat for 1 minute. Mix in the lemon and orange juice. In a medium-sized, heavy-based saucepan, cook the mixture over low heat until the white butter/sugar particles dissolve, stirring constantly. There may be traces of white foam sitting on top of mixture, which will eventually dissolve and disappear once thickened. Increase to medium heat and cook for about 10-15 minutes, until the mixture reaches 170° F on a candy thermometer or thickens enough to coat the back of a spoon. Do not let mixture boil. Remove curd from heat. Stir in orange zest. Pour curd into a medium-sized bowl and cover the surface of the curd with plastic wrap to prevent a pudding-like skin from forming. Chill curd in refrigerator (it will thicken further as it chills).

Makes 2 cups