Whisk together white rice flour, unsweetened cocoa powder, brown rice flour, tapioca starch, potato starch, xanthan gum, baking powder, and salt. Set aside.
Mix oil, chocolate and first measurement of sugar in the bowl of standing mixer fitted with the paddle attachment. Add eggs, one at a time, mixing until completely incorporated. Blend in vanilla.
Add the dry ingredients (except powdered sugar) to the chocolate mixture, blending until incorporated. Cover and chill dough several hours or overnight.
When ready to bake, preheat oven to 350 degrees F. Scoop dough creating balls about 1" large for each cookie. Dough will be sticky. Roll dough ball in powdered sugar. Place cookies about 2 inches apart on a silpat or parchment-lined baking sheet.
Bake 10-12 minutes in a 350 F degree oven. Allow cookies to set on cookie sheet for a couple of minutes before using a thin metal spatula to move them to a cooling rack.