In a large saucepan over medium heat, combine the cream, egg yolks, lemon juice, and lemon zest. Cook, whisking frequently, until the mixture thickens, about 5 to 8 minutes (but watch carefully, you do not want it to curdle).
Remove from heat and add the butter and lemon extract. Sprinkle with xanthan gum and whisk quickly to combine. Stir in the sweetener to taste (I like mine rather tart).
Whisk in the 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).