Preheat your oven to 350°F (175°C). Grease a 10x15-inch jelly roll pan or a similar sized baking sheet with cooking spray or butter.

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 is completely melted and the mixture comes to a gentle boil. Remove from heat immediately.

Pour the hot chocolate mixture over the dry ingredients and stir until just combined. In a separate small bowl, whisk together the eggs, buttermilk, and vanilla extract. Add this to the chocolate mixture and stir until smooth and well combined.

Pour the batter into your prepared pan, spreading it evenly to all corners. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean and the cake springs 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 immediately stir in the vanilla extract.

Gradually whisk in the powdered sugar until the icing is smooth and glossy. If using nuts, stir them in at this point.

As soon as the cake comes out of the oven, pour the warm icing over the hot cake, spreading it evenly with a spatula.

Allow the cake to cool completely in the pan before cutting into squares. Serve at room temperature.