Pre-heat the oven to 350º F and grease a 16x10x1 sheet cake pan.
In a large mixing bowl, whisk together the flour, xanthan gum, baking soda, sugar, and salt.
In a medium saucepan, melt the butter.
Then add the cocoa powder and water, whisking together.
Bring this mixture to a boil over low to medium heat.
Then pour the cocoa mixture over the dry ingredients.
Add the buttermilk, eggs, and vanilla extract, mixing well 'til everything is well-combined.
Pour the cake batter into the greased sheet cake pan.
Bake at 350º F for 25-30 minutes, or until a toothpick or cake tester inserted into the center comes out clean.