Preheat oven to 325ºF. Boil or bake sweet potatoes until tender and set aside to cool. In a large mixing bowl, combine the butter, sweet potatoes, sugars, and flour. Using a metal spoon, mash all ingredients until well blended. Add eggs, heavy whipping cream, lemon juice and vanilla and continue to whip until the batter is smooth and creamy. Season with cinnamon and nutmeg and pour into baked pie shell. Place on center oven rack and bake approximately forty five minutes. Remove and allow to cool.