Combine egg yolks, whole eggs, sugar, and lemon juice in medium-size heavy nonreactive saucepan. Cook over low heat, stirring constantly with wooden spoon, until thick enough to coat back of spoon, about 15 minutes. Remove from heat; transfer to heat-proof bowl.Advertisement
Stir in butter, one piece at a time, until fully incorporated. Stir in lemon zest. Press plastic wrap onto surface of lemon curd to prevent a skin from forming, and refrigerate until ready to use.