Preheat your oven to 350°F (175°C). While it warms up, grab a baking sheet measuring 18 inches by 26 inches and grease it generously with butter and dust it with some flour. This ensures your cake won’t stick!
In a medium saucepan over medium heat, combine butter, cocoa powder, and water. Stir occasionally until the butter melts and the mixture comes to a gentle boil. Once boiling, remove it from the heat.
In a large mixing bowl, combine flour, sugar, and baking soda. Mix these dry ingredients well before pouring in the hot butter mixture from the saucepan. This helps to evenly distribute the flavors.
Next, add in the eggs, buttermilk, and vanilla extract. Stir until everything is well-combined and the batter is smooth. You want a lovely consistency that’s easy to pour.
Pour the batter into the prepared baking pan, spreading it evenly. Don’t worry if it seems a little thin; it’s supposed to be that way!
Place the pan in the preheated oven and bake for 18 to 20 minutes. To check for doneness, insert a toothpick in the center; if it comes out clean, you’re all set!
While the cake is baking, let’s prepare the frosting. In another saucepan, combine butter, cocoa, and buttermilk. Bring it to a boil, stirring continuously to blend the flavors.
Once boiling, remove from heat and gradually whisk in the powdered sugar until the frosting is smooth and glossy. This frosting should be thick but still pourable.
Add the vanilla and mix well. Fold in the pecans or walmuts, if using. This adds a delightful crunch to your frosting!
After your cake is out of the oven, allow it to cool for a few minutes before pouring the warm frosting over the top. Spread it evenly and let it set before cutting into squares.