Preheat oven to 350 F. Spray a 10x15-inch pan with cooking spray.

Combine the butter, water and 5 tablespoons cocoa in a saucepan on medium heat. Bring to a boil and remove from heat.

In a mixing bowl, combine the flour, xanthan gum, sugar, baking soda and salt.

On medium speed, mix in the buttermilk and eggs. After 1 minute, add the cocoa mixture and mix until well combined.

Pour batter into prepared pan and bake for 20-25 minutes or until toothpick inserted in center comes out clean.

While cake is baking, in a large saucepan combine the milk, 5 tablespoons cocoa and butter. Bring to a boil and remove from heat. Stir in powdered sugar and vanilla, mixing until blended.

Spread icing over cake after removing from oven. Cool.