Chocolate Buttermilk Texas Sheet Cake

Preheat oven to 350 F and grease a cookie sheet pan (13×18''). Set aside.
Add butter, water and cocoa powder to a small saucepan over medium heat. Bring mixture to a boil over medium heat.
In a large bowl whisk together the sugar, flour, baking soda and salt. As soon as butter/cocoa mixture comes to a boil remove from heat and pour over the dry ingredients. Whisk together until all combined.
In a measuring cup or small bowl whisk the eggs, buttermilk and vanilla. Mix into the cake batter.
Pour batter into prepared pan. Bake for 17-22 minutes, or until edges are pulling away. When there is about 7-10 minutes left before the cake is done baking, start the chocolate frosting.
Add butter, cocoa powder and milk to the same small saucepan and bring to a boil over medium heat. Remove from heat and pour into a medium mixing bowl with vanilla, pinch of salt and powdered sugar. I like to mix with electric beaters on low to make sure it's smooth.
Pour frosting over cake after it's cooled for 3-5 minutes, and use a spatula to spread evenly over the cake. The cake needs to be warm enough that the frosting can spread easily over the cake.