Preheat oven to 350℉ and prepare 13"x18" pan with a layer of butter covered with a light layer of flour.
Melt butter, cocoa powder, and water over medium heat. Bring to a boil then allow to cool for 1-2 minutes.
Add flour and sugar to large bowl and mix.
Pour chocolate mixture into large bowl and mix.
Add buttermilk, eggs, vanilla extract, baking soda, salt and mix.
Bake for 20 minutes.
Start making frosting 8-10 minutes before the cake is done.
Melt butter, cocoa powder, and milk on medium heat. Bring to a boil.
Add powdered sugar, vanilla extract, and chocolate mixture together in medium bowl.
Pour warm frosting over hot cake.