Preheat oven to 375 degrees F (190 degrees C). Grease and flour a 9x13 inch pan. Mix together the sugar, flour, cocoa, baking powder, baking soda and salt. Set aside.

In a large saucepan bring butter and water to a boil. Remove from heat. Stir in flour mixture. Pour in buttermilk, eggs and vanilla; mix thoroughly.

Pour cake batter into prepared 9x13 inch baking pan. Bake in preheated oven for 25 minutes or until a toothpick inserted in center comes out clean. Remove cake from oven and frost immediately.