Preheat your oven to 350°F (175°C). Grease a 13x18 inch baking sheet.
In a large bowl, combine flour, sugar, baking soda, and salt. Set aside.
In a saucepan, melt the butter. Add cocoa powder and boiling water, and bring to a boil. Remove from heat.
Pour the cocoa mixture over the flour mixture and stir until well combined.
Add buttermilk, beaten eggs, and vanilla extract to the batter. Mix well.
Pour the batter into the prepared baking sheet and spread evenly. Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the frosting. In a saucepan, melt the butter. Add cocoa powder and milk, and bring to a boil. Remove from heat.
Stir in powdered sugar and vanilla extract until smooth. Add chopped pecans if using.
Pour the frosting over the hot cake and spread evenly. Let the cake cool before serving.