Grease a 15 x 10 x 1 inch baking pan. Set pan aside. In a large mixing bowl stir together flour, sugar, baking soda, and salt. Set mixture aside.In a saucepan combine 1 cup butter (or margarine), water, and 1/3 cup cocoa powder. Bring just to a boil, stirring constantly. Remove from heat. Add chocolate mixture to flour mixture. Beat with an electric mixer on medium to high speed until thoroughly combined. Add eggs, 1/2 cup buttermilk, and 1 1/2 teaspoons vanilla. Beat for I minute (batter will be thin ).Pour batter into the prepared baking pan. Bake in a 350F oven for about 25 minutes or until a wooden toothpick inserted near the center comes out clean. Cool slightly in pan on a wire rack.Meanwhile, for frosting, in a saucepan combine 1/4 cup butter (or margarine), 3 tablespoons cocoa powder, and 3 tablespoons buttermilk. Bring to boil, then remove from heat. Stir in powdered sugar and 1/2 teaspoon vanilla until smooth. If desired, stir in pecans. Pour warm frosting over warm brownies, spreading evenly. Cool completely. Cut into bars.