Make the Cake: Preheat oven to 350 degrees F. Grease a 12x18x1 pan with nonstick cooking spray. Set aside.
Whisk together granulated sugar, white rice flour, sweet rice flour, cornstarch, baking soda, and salt.
Melt the butter over medium heat in a small saucepan over medium heat. Add cocoa powder and stir to combine. Reduce heat to low and allow mixture to cook for 3 minutes. Stir in boiling water and cook for 30 seconds. Remove from heat and allow to cool for 30 seconds.
Pour mixture over dry ingredients. Stir to combine. (You can use a balloon whisk for this.) Blend until smooth. Add buttermilk, eggs, and vanilla. Mix until a batter forms. Pour batter evenly into prepared pan.
Bake for 25 minutes or until cake springs back to the touch. Remove cake from the oven.
Make the Frosting: Melt butter in a small saucepan over medium heat. Add cocoa powder. Stir to combine. Turn off heat. Add the powdered sugar, milk, and vanilla. Stir until smooth. Pour over warm cake.