In a medium saucepan, whisk eggs with 1 tablespoon water until light and frothy. Put heat on low. Continuing to whisk, add lemon juice. Raise heat just slightly and begin adding butter, 1 tablespoon at a time. As it incorporates you can add more at a time, continuing to whisk briskly, until mixture thickens. Remove from heat and add cayenne if using and a few drops of vanilla, and taste to adjust lemon juice, and whisk well to incorporate. Use right away, or allow to rest until needed.