1 cup plus 1 TB flour consisting of -
1 part rice flour
1 part tapioca
1 part potato starch
1/3 cup cocoa powder
1 tsp salt
1 tsp guar gum (If using xanthum gum, reduce - perhaps 1/4 tsp?)
2/3 cup sugar
1/2 tsp baking soda
Mix in -
1/3 cup plus 1 TB oil (could use butter, if desired)
2/3 cup milk / buttermilk / coffee (I used milk)
1 tsp vanilla
Batter will be thin - think something like yogurt. Bake a little under 30 minutes at 350. It should double in height, and may crack on the top. Test with a toothpick. The original recipe suggested freezing it after it cooled for 5 minutes to help keep it moist, but it's edible out of the pan.
For low altitude instructions, add 2 TB sugar, remove 2 TB flour, reduce the liquid by 10%, slightly increase the leavening (1/4 tsp baking powder, perhaps?).