Egg whites should be at room temperature. Put a dozen eggs in a bowl of warm water or separate the whites and let them set for about 30 minutes.
Preheat oven to 350°
In a medium bowl, whisk together the powdered sugar, Free Indeed gluten-free flour, xanthan gum, and salt.
In the bowl of the electric mixer, fitted with a whisk attachment beat together the egg whites, cream of tartar, and vanilla extract until foamy. Slowly add the granulated sugar, beating until stiff peaks form. Do not over beat.
Sift about 1/4 of the flour mixture over the egg whites and fold it into the batter with a rubber spatula. Continue adding the flour by fourths by sifting and folding until it's all incorporated.
Spoon the mixture into an ungreased angel food cake pan and smooth the top.
Bake for 45 - 50 minutes. The cracks in the top of the cake should be dry and the top should spring back when touched. Turn the cake upside down and cool for at least an hour in the pan.