Pre-heat the oven to 350° F.
In a large mixing bowl, whisk together the dry ingredients, including the sugar, powdered sugar, flour, xanthan gum, and salt.
In a separate large bowl with a mixer, beat the egg whites and cream of tartar together until they form soft peaks.
Gently fold the dry ingredients into the egg whites by hand.
Add the vanilla extract, and gently mix.
Spread the batter into an ungreased tube pan, and bake at 350° F for about 1 hour, or until a cake tester comes out clean. It's very important that your cake pan be un-greased because you want your cake to stick to the sides and bottom of the pan. You'll see why in the next step.
When your cake comes out of the oven, immediately turn it upside down to cool for about 1 hour. You can put it on a wire rack, or you can turn it upside down, placing the tube part of your pan on the neck of a bottle or on a can.
Once your cake has cooled completely, run a knife around the edge, and remove the cake from the pan.
Then slice and serve, preferably with fresh berries and whipped cream.