Preheat the oven to 350°F. Lightly spray an 18” x 13” pan with cooking spray.
In a small saucepan over medium heat, add the butter, water, and cocoa powder. Bring to a boil. Remove from heat. Set aside.
In a large bowl add flour, sugar, baking soda, and salt. Whisk to combine. Add the chocolate mixture. Mix well. Add eggs, vanilla, and buttermilk. Mix until combine.
Pour batter into prepared pan. Bake for 15-20 minutes, or until toothpick inserted in the center comes out clean. (All ovens vary, mine finishes baking by 15 minutes, but yours might need the full 20 minutes, so keep an eye on it).
Cool the cake for about 3 minutes before pouring the frosting over it.
Once the cake is halfway done baking begin making the frosting. In a small saucepan over medium heat, add the butter and heat until melted. Once fully melted add the cocoa powder and stir until smooth.
Remove from heat. Add the milk, vanilla, and powdered sugar. Stir until combined and smooth. Pour the frosting over the cake and spread quickly before it sets.
**Bake in a regular sized baking sheet for 20-22 minutes.