Combine the first 3 ingredients in a saucepan over medium heat, stirring with a whisk. Cook until sugar dissolves and mixture is light in color (about 3 minutes). Stir in lemon juice and butter; cook for 5 minutes or until mixture thinly coats the back of a spoon, stirring constantly with a whisk. Cool. Cover and chill (the mixture will thicken as it cools).
Note: Lemon Curd can be stored in the refrigerator for up to 1 week. You can easily double the recipe and freeze half of it in a heavy-duty zip-top plastic bag. Thaw in the refrigerator, and use within 1 week of thawing.