Prepare the crust: In a medium bowl, whisk butter, sour cream, and xylitol until smooth. Stir in flour until a ball of dough forms. Press dough evenly into a greased 9x13 baking pan. Prick dough all over with a fork. Bake crust at 350F 20 minutes, or until lightly browned.

While crust bakes, prepare the filling: In a medium bowl, whisk eggs, lemon juice, zest, xylitol, and flour until smooth. Pour evenly over pre-baked crust. Return pan to oven and bake at 350F for an additional 20 minutes, or until filling is set. (filling may puff up a little bit---don't worry, it will deflate while it cools) Cool bars on a wire cooling rack completely before cutting and sprinkling with extra powdered xylitol.