Grease and flour a 15X10X1-inch or jelly roll pan {I used a cookie sheet with sides}, set aside.
In a large bowl, combine flour, sugar, baking soda, and salt; set aside.
In a medium saucepan combine 1 cup butter, ⅓ cup cocoa, and 1 cup of water. Bring mixture to a boil, stirring constantly. Remove from heat.
With an electric hand-held mixer on medium speed, beat chocolate mixture into the dry mixture until thoroughly blended. Add eggs, ½ cup of buttermilk, and 1 ½ teaspoons of vanilla. Beat for 1 minute (batter will be thin). Pour batter into the prepared pan.
Bake in a 350° oven about 22 to 25 minutes or until a wooden pick or cake tester inserted in center comes out clean.
Pour warm chocolate frosting over the warm cake, spreading evenly. Place cake in pan on a wire rack; cool thoroughly before cutting.
Makes 24 servings.