Preheat the oven to 350. Grease two round 8" or 9" cake pans with coconut oil or butter, or spray them with baking spray. Line the bottoms with parchment paper, grease again lightly and sprinkle with cocoa powder to coat the inside. Tap out the excess and set aside.
In a large bowl, sift together the brown rice flour, potato starch and tapioca flour and make sure everything is well mixed. Measure out 2 1/2 cups of this mixture to use in the recipe and save the rest for another use.
In a large bowl or in a stand mixer, add the flour blend you made, sugar, baking soda, baking powder, xanthan gum and salt. Whisk or mix together to combine.
In a 4-cup measuring cup or a medium size bowl, measure in the buttermilk and water. Add the egg, oil and vanilla and whisk to break up the egg.
Add the buttermilk mixture to the flour mixture and blend with a hand mixer, whisk or in your stand mixer until no lumps of flour remain.
Divide the batter between the two pans and bake on the center rack, rotating half way through, until a toothpick comes out clean--25 to 30 minutes. Don't overbake your cakes or they will be dry. Start checking them 5 to 7 minutes before the end of the baking cycle. If you use 8" pans your cakes may take closer to 40 minutes to bake.
Once baked, let cool in the pans 5 minutes then turn out on a wire rack to finish cooling completely.