In a 1-quart saucepan combine sweet potatoes, water, sugars, 2 Tbsp of the butter; the vanilla and lemon juice and rind. Cover and cook over medium heat for 20 minutes, stirring occasionally; uncover and cook until potatoes are fork tender, about 10 minutes. Add the remaining butter, stirring until completely melted. Cook uncovered until sauce is thick, about 2 minutes more. Makes about 1 Cup.