Preheat the oven to 350℉. Spray a 17x12x1-inch rimmed baking sheet with nonstick spray.
In a medium saucepan, add the butter, water and cocoa powder. Bring the mixture to a boil, then pull off the heat and set aside.
In a large bowl, whisk together the flour, sugar, baking soda and salt. Whisk in the buttermilk and eggs. Then whisk in the hot mixture until well-blended.
Scrape the batter into the prepared pan and spread evenly. Bake for 25 minutes, or until the cake tests done with a toothpick inserted into the middle coming out clean.
PREPARE THE FROSTING (while the cake is still warm):
In a medium saucepan, add the butter, milk and cocoa powder and bring to a boil. Remove the pan from heat and stir in the vanilla. Whisk in the powdered sugar until well blended. Pour the frosting onto the warm cake and spread evenly to cover. Sprinkle the cake with nuts, if using.
Notes
Sifting the powdered sugar will prevent lumps from forming in the frosting.