Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan and set aside. In a large mixing bowl, whisk together flour, sugar, cocoa powder, baking soda, and salt until well combined.

In a medium saucepan, combine butter and water. Heat over medium heat until the butter melts and the mixture comes to a gentle boil. Remove from heat and pour this hot mixture over the dry ingredients. Stir until just combined.

In a small bowl, whisk together buttermilk, eggs, and vanilla extract. Add this to the chocolate batter and stir until smooth and well combined. The batter will be thin - this is normal!

Pour the batter into your prepared pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. While the cake bakes, prepare the frosting.

In a medium saucepan, combine butter, cocoa powder, and milk. Heat over medium heat, stirring constantly, until the butter melts and the mixture is smooth. Remove from heat and whisk in powdered sugar and vanilla until the frosting is smooth and creamy.

Pour the warm frosting over the hot cake as soon as it comes out of the oven. Use a spatula to spread it evenly across the surface. If using nuts, sprinkle them over the frosting while it's still warm. Let the cake cool completely before cutting into squares.