Preheat your oven to 350°F (175°C). Grease a 10x15-inch jelly roll pan or similar-sized baking sheet. In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt until well combined.

In a medium saucepan, combine the butter, water, and cocoa powder. Heat over medium heat, stirring constantly, until the butter melts and the mixture comes to a gentle boil. Remove from heat and let cool slightly for 2-3 minutes.

Pour the warm chocolate mixture over the dry ingredients and stir until just combined. Add the buttermilk, eggs, and vanilla extract, mixing until the batter is smooth and no streaks remain. Be careful not to overmix.

Pour the batter into the prepared baking sheet, spreading it evenly to the edges. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean. The cake should spring back when lightly touched.

While the cake is baking, prepare the icing. In a medium saucepan, combine the butter, cocoa powder, and milk. Heat over medium heat, stirring constantly, until the butter melts and the mixture begins to boil. Remove from heat and whisk in the vanilla extract and powdered sugar until smooth. Stir in the nuts if using.

As soon as the cake comes out of the oven, pour the warm icing over the hot cake, spreading it evenly with a spatula. The icing will set as the cake cools, creating that signature fudgy texture.