Preheat oven to 400ºF. Grease a 9×13″ pan.
Sift together flour and sugar; set aside.
In a saucepan over high heat, bring butter, cocoa, and water to a boil, then pour over the sugar and flour mixture. Mix well.
Add buttermilk and eggs, baking soda, and vanilla to flour mixture and mix well.
Pour into prepared pan. Bake for 25 minutes or until a toothpick comes out clean when inserted in the center.