Grease an 8 inch spring form pan and preheat oven to 350 degrees.
Place your egg whites and cream of tartar in a stand mixer and mix until stiff peaks form. Set aside.
Place your yolks in another bowl and whisk with vanilla extract, liquid stevia and lemon zest.
In another bowl whisk together the remaining dry ingredients.
Combine the yolks and dry ingredients together until smooth.
In small portions a little at a time, fold the egg whites into the yolk and flour mixture, be careful not to over mix and deflate your egg whites.
Drizzle the melted, cooled butter on the side of the bowl to slowly fold it into the batter.
Pour batter into pan and bake 30 minutes or until golden on top and a toothpick in center comes out clean.
Allow to cool completely before removing and adding optional toppings.