Grease a 15X10X1-inch or jelly roll pan or a 13X9X2-inch baking pan; set aside. In a large bowl, combine flour, sugar, baking soda, and salt; set aside.
In a medium saucepan combine 1 cup butter, 1/3 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, buttermilk, and vanilla. Beat for 1 minute (batter will be thin). Pour batter into the prepared pan.
Bake in a 350° oven about 25 minutes for the 15X10-inch pan or 35 minutes for the 13X9-inch pan, 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.
In a medium saucepan combine 1/4 cup butter or margarine, 3 tablespoons unsweetened cocoa powder, and 3 tablespoons buttermilk. Bring to a boil. Remove from heat; add 2 1/4 cups sifted confectioners' sugar and 1/2 teaspoon vanilla. Beat until smooth. If desired, stir in 1/2 cup coarsely chopped pecans.