Preheat oven to 350 degrees. Grease two 9-inch pans, a 13 x 9 pan, or line cupcake pans with cupcake liners.
In a large bowl combine white rice flour, tapioca flour, potato starch, guar gum, baking powder, and sugar.
Place cocoa in a small bowl. Pour the hot water over the cocoa.
Add oil, eggs, and vanilla to the cocoa mixture.
Add the liquids to the flour mixture and beat for 2 –3 minutes on high.
Pour batter into 2 greased 9-inch cake pan. Bake at 350 for 30 - 35 minutes (30 - 40 minutes for a 13 - 9 pan and 18 - 22 minutes for cupcakes) or until an inserted toothpick comes out clean.