Preheat an oven to 350 degrees. Spread the butter inside a ramekin for baking until there is a thick, visible layer of butter across the entire inside. Set the buttered ramekin in your refrigerator to stay cold.
In a bowl, whisk together the almond flour, lemon zest, salt, first amount of stevia, xanthan gum, cream, vanilla and just the yolk of your egg. Set the egg white in a stand mixer.
In your stand mixer, whisk together the egg white with the cream of tartar and second amount of stevia until your have a thick, glossy meringue.
Use a spatula to fold the meringue into your lemon mixture, being careful not to deflate the air bubbles in the meringue.
Pour the batter into your cold ramekin. Rotate the ramekin to level the batter without deflating the bubbles. Then, use the tip of your thumb to create an indented ring inside the entire edge of your batter, as pictured.
Set the ramekin on a sheet tray and bake for 20-22 minutes. Let the souffle rest for a few minutes before eating. It will rise very high during baking and gently deflate once cooled.