Step 1

Preheat the oven to 325°F and lightly grease a 9x13 pan. Set aside.

Step 2

In a large mixing bowl, whisk the flour, xanthan gum (omit if your flour blend already contains xanthan gum), brown sugar, cocoa, baking soda, and salt.

Step 3

Add the hot brewed coffee and oil to the dry ingredients, then stir the ingredients using a spatula or spoon until combined.

Step 4

Add the buttermilk and eggs and stir the ingredients with a spatula or spoon until well combined.

Step 5

Pour the batter into the prepared cake pan and spread it evenly with a spatula.

Step 6

Bake the cake until it springs back if gently touched or a toothpick inserted near the center of the cake comes out mostly clean, with a little crumb or two, about 35-40 minutes.

Step 7

Allow the cake cool completely in the pan, then frost as desired.

Step 8

Add the butter to a standing mixer and beat it until it's whipped and smooth. Add the powdered sugar, vanilla, and milk and continue to beat the mixture until it's light and fluffy about 2 minutes.