Preheat your oven to 350°F (175°C). Grease a baking sheet.
In a large mixing bowl, combine flour, sugar, baking soda, and salt.
In a saucepan, melt the butter with water and cocoa powder. Bring to a boil, then remove from heat.
Pour the cocoa mixture into the dry ingredients and mix well.
Add buttermilk, beaten eggs, and vanilla extract. Mix until smooth.
Pour the batter into the prepared baking sheet and bake for 20 minutes.
While the cake is baking, prepare the icing. In a saucepan, melt the butter with cocoa powder and milk. Bring to a boil, then remove from heat.
Stir in powdered sugar and vanilla extract until smooth. Add chopped pecans if using.
Pour the icing over the hot cake and spread evenly. Let it cool before serving.