Fresh lemons are a must in this recipe and give it unbeatable flavor. For a lime-curd variation, substitute lime rind and juice.
Combine the first 3 ingredients in a medium, heavy saucepan, stirring with a whisk. Stir in juice and eggs; bring to a boil over medium heat, stirring constantly with a whisk. Reduce heat, and simmer 1 minute or until thick, stirring constantly. Remove from heat; add butter and lemon rind, stirring gently until butter melts.
Spoon mixture into a medium bowl; cool. Cover and chill at least 6 hours or overnight (mixture will thicken as it cools).