Lightly beat egg yolks in a bowl, set aside.
In a medium saucepan stir together the sugar, cornstarch and lemon zest, mix well. Add in the lemon juice and water. Cook over medium heat stirring gently until mixtures begins to thicken and bubble.
Slowly pour half of the lemon mixture into the eggs, stirring briefly to temper the eggs. Then pour the egg mixture back into the saucepan, bring to a gentle boil over medium heat, stirring gently and continue to cook for 2 minutes. Remove from heat, add the butter, stirring until melted.
Transfer to a storage container. Chill for at least an hour. Stores in refrigerator for 1 week or in freezer for up to two months. Thaw in refrigerator before serving.
Makes 2 cups.