Separate the eggs and let the whites sit in your mixing bowl (glass or metal, not plastic) for about 30 minutes so they come to room temperature.
Be sure your bowl is very clean and there is absolutely no yolk in the whites. It is best to separate each egg into a small bowl, then add it to the rest of the whites. That way if a bit of yolk gets in with the white, you can save the egg for another purpose and not ruin the whole batch.
Preheat oven to 350°
In a medium bowl, whisk together the powdered sugar, potato starch, tapioca starch, millet flour, xanthan gum, and salt.
In a large mixing bowl, beat together the egg whites, cream of tartar, and vanilla extract until foamy. I used my KitchenAid whisk attachment.
Slowly add the granulated sugar, 2 tablespoons at a time, beating until stiff peaks form. Do not over beat.
Fold in the flour mixture about one fourth at a time, making sure you scrape the bottom of the bowl when you fold. Do not mix, just gently fold until all the flour mixture is moistened.
Spoon the mixture into an ungreased angel food cake pan (one without a non stick coating) and smooth the top.
Remove air pockets by gently cutting through the center of the batter with a thin metal spatula (icing spreader) or knife.
Bake for 35 minutes, then check for doneness. A wooden skewer inserted in the center should come out dry. Also, the cracks in the top of the cake should be dry and the top should spring back when touched. Mine took 45 minutes.
Turn the cake upside down and cool for at least an hour in the pan. Most tube pans have legs to keep the top of the cake off the counter. If yours does not, invert onto a bottle inserted into the tube.
When cool, run your metal spatula or a knife around the sides to loosen. Remove the outer part of the pan. Run your spatula under the cake to loosen it from the bottom and remove to a cake plate.
Recipe by Gluten-Free Homemaker at https://glutenfreehomemaker.com/angel-food-cake/