Preheat your oven to 350°F (175°C), then grease and flour a 13x18-inch jelly roll pan.

In a large bowl, whisk together the flour, granulated sugar, baking soda, and salt.

Melt butter in a saucepan, then stir in cocoa powder and water, bringing it to a boil before removing from heat.

Pour the hot cocoa mixture over the dry ingredients and stir until just combined.

Whisk buttermilk, eggs, and vanilla in a separate bowl, then add to the batter and stir until smooth.

Pour batter into the prepared pan and bake for 20-25 minutes, or until a wooden skewer comes out clean.

While the cake bakes, melt butter in a saucepan, then stir in milk and cocoa powder, bringing to a boil and then removing from heat.

Immediately add powdered sugar and vanilla extract to the hot cocoa mixture and whisk until smooth; stir in pecans if desired.

As soon as the cake is out of the oven, pour the warm frosting over it, spreading evenly.

Allow the cake to cool completely before cutting and serving to allow the frosting to set.