Preheat the oven to 350 degrees Fahrenheit. Grease a 9 x 13 baking pan.
In a medium bowl, whisk to combine sweet white rice flour, millet flour, tapioca starch, sorghum flour, salt, baking soda, and cocoa powder. In another medium bowl, beat together with electric mixer butter, coconut sugar, and egg.
Slowly add dry ingredients to the butter mixture, stirring to incorporate. Add the water, vanilla extract, and vinegar. Stir until completely combined. Pour the batter into prepared pan and bake for 20 to 22 minutes or until a toothpick comes out clean. Allow cake to cool completely prior to frosting.
For the frosting:
In a medium bowl combine butter, powdered sugar, cocoa powder, and coffee. Using electric beater, mix until smooth.
Notes
Adapted from Bon Appetit and my Chocolate Cake with Pumpkin Spice Frosting*You can blitz unrefined cane sugar in the blender to make your own refined sugar-free powdered sugar if desired. **If you prefer not to use coffee, sub the called for amount with unsweetened almond milk or milk of choice.