Preheat oven to 350 degrees F. and spray a 13 x 17 1/2 inch jelly roll pan with cooking spray. You can also line with tin foil if you'd like.

To prepare cake, place 2 sticks of butter in a medium saucepan over medium heat to melt. Stir in water then whisk in cocoa powder until smooth. Stir until mixture comes to a boil then remove from heat and let cool for 10 minutes.

Place flour, sugar, baking soda and salt into a large mixing bowl, stirring to combine. Place buttermilk, eggs and vanilla into a small bowl, whisk then stir into flour mixture. Pour warm chocolate mixture into bowl, slowly. I do this in three phases, stirring in between each. Pour cake batter into jelly roll pan and bake for 18-20 minutes until baked through.

Start preparing icing when there's 5 minutes of bake time left on the sheet cake. Place milk and cocoa powder into a large saucepan over medium heat, whisking to combine. Stir in softened butter and bring mixture to a boil. When comes to a boil, turn off heat then slowly add powdered sugar whisking very well until icing is a nice consistency. Note, this icing sets up if left alone too long. Needs to be poured over cake as soon as its done. Pour icing over warm cake spreading evenly over sides. Let cake cool for 20 minutes before cutting into squares. Note, using a plastic knife to cut squares helps cut without cake sticking.