In a large bowl, place the flour, xanthan gum, cocoa powder, salt, baking soda and sugar, and whisk to combine well. Create a well in the centre of the dry ingredients and add the sour cream, oil, eggs and water, mixing to combine after each addition. The batter should be very thickly pourable. Scrape the batter into the prepared pan, smooth into an even layer and place in the center of the preheated oven.