Preheat the oven to 350 °F. Prepare a 9x9 inch glass baking pan by lining it with parchment paper.
Combine the melted butter for the Crust, the flour, Crust sweetener, vanilla, and Crust xanthan gum, stirring until mixed. Press the crust into the prepared pan. Bake for 15-17 minutes or until lightly golden. Remove and set aside.
While the crust bakes, stir together the Filling butter, sweetener, lemon powder, and xanthan gum in a small saucepan. Whisk in the egg yolks. Place the pan over medium heat, stirring continuously, for 1-2 minutes, until thick and smooth. Stir in the lemon oil.
Pour the filling over the baked crust and tilt the pan to spread it evenly. Bake for 15-20 minutes. Remove, cool for ten minutes, and then place in the freezer to cool for about ten minutes to solidify.
Remove the bars from the freezer and slice them into 9 portions. Freeze the slices individually or serve them immediately.