In a large saucepan over medium heat, combine cream, egg yolks, lemon juice, and lemon zest. Cook, whisking frequently, until mixture thickens, about 5 to 8 minutes (but watch carefully, you do not want it to curdle).
Remove from heat and add butter and lemon extract. Sprinkle with xanthan gum and whisk quickly to combine. Stir in sweetener to taste (I like mine rather tart).
Whisk in sour cream until no lumps remain. Pour into pie shell and refrigerate for at least 4 hours to set - the longer, the better, so overnight is best if you can wait that long. (It's also awesome if you pop it in the freezer for an hour after being refrigerated).
Top with lightly sweetened whipped cream.
Serves 10. Each serving has 3.92g NET CARBS. (this includes the pie crust)Food energy: 330kcal Total fat: 31.05g Calories from fat: 279 Cholesterol: 142mg Carbohydrate: 5.74g Total dietary fiber: 1.82g Protein: 5.77g Sugar alcohols (erythritol): 18g