Preheat the oven to 350F. Coat two 9-inch cake pans with cooking spray and set aside.
In a large bowl, combine the flour, sugar, cocoa powder, baking soda, xanthan gum, and salt, and blend.
In a medium bowl, combine the milk, oil, vinegar, and vanilla.
Pour the wet ingredients into the dry ingredients. Stir until smooth, scraping the bottom and sides of the bowl.
Pour into greased cake pans, and place in the oven.
Bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven, and let them cool for 5 minutes, then move them to a wire rack to cool completely.
While the cakes are cooling, make the frosting. (I did mine in my food processor, but you can also make it in a large bowl with an electric mixer.)
Beat the butter until light and fluffy.
Add the vanilla, cocoa powder, and powdered sugar.
Add the coconut milk or creamer slowly until it's completely mixed.
If the frosting is too thick, add more creamer 1 tablespoon at time. If it's too runny, add more powdered sugar 1 tablespoon at a time.
Frost the cakes after they are completely cooled.