Preheat your oven to 350°F (175°C). Grease a 15x10-inch jelly roll pan or standard sheet pan. In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.

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

Pour the chocolate mixture over the dry ingredients and stir until just combined. In a small bowl, whisk together the eggs, buttermilk, and vanilla extract. Add this to the batter and mix until smooth. Pour into the prepared pan and bake for 20-25 minutes, or until a toothpick inserted in the center comes out clean.

While the cake bakes, prepare the icing. In a medium saucepan, combine the butter, milk, and cocoa powder. Bring to a boil, then remove from heat. Whisk in the powdered sugar and vanilla until smooth. Stir in pecans if using.

Pour the warm icing over the hot cake immediately after it comes out of the oven. Spread evenly with a spatula. Allow the cake to cool completely on a wire rack before cutting into squares.