Preheat oven to 350 degrees F. Grease a 15x10x1-inch or a 13x9x2-inch baking pan; set aside. In a medium bowl, combine flour, sugar, baking soda, and salt; set aside.

In a medium saucepan combine water, butter, and cocoa powder. Bring mixture just to boiling, stirring constantly. Remove from heat. Add the cocoa mixture to the flour mixture. Beat with an electric mixer on medium speed until combined. Add the eggs, buttermilk, and vanilla. Beat for 1 minute (batter will be thin). Pour batter into prepared baking pan.

Bake about 25 minutes for the 15x10x1-inch pan, about 35 minutes for the 13x9x2-inch pan, or until a wooden toothpick inserted in the center comes out clean.

Pour warm Chocolate-Buttermilk Frosting over the warm brownies, spreading evenly. Cool completely in pan on a wire rack. Cut into bars. Makes 24 brownies.

Chocolate-Buttermilk Frosting: In a medium saucepan combine 1/4 cup butter, 3 tablespoons unsweetened cocoa powder, and 3 tablespoons buttermilk or sour milk. Bring mixture to boiling. Remove from heat. Add 2-1/4 cups powdered sugar and 1/2 teaspoon vanilla. Beat with an electric mixer on medium to high speed until smooth. If desired, stir in 3/4 cup coarsely chopped pecans.

Note: To make 1 cup sour milk, place 1 tablespoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1 cup total liquid; stir. Let mixture stand for 5 minutes before using. Use 1/2 cup sour milk in the brownies and 3 tablespoons sour milk in the frosting. Discard remaining sour milk. Or reconstitute dry buttermilk by following the directions on the powdered buttermilk container.