In mixing bowl, combine flour, sugar, baking soda, and salt. Set aside.
In medium saucepan, combine 1 cup butter, 1 cup water, and 1/3 cup unsweetened cocoa.
Bring mixture just to boiling, stirring constantly.
Remove from heat.
Add chocolate mixture to dry ingredients and beat until thoroughly combined.
Add eggs, the 1/2 cup buttermilk, and 1 1/2 teaspoons vanilla. Beat for 1 minute (batter will be thin).
Pour batter into greased and floured 15x10x1 baking pan. Bake at 350F for about 25 minutes or until toothpick inserted into the center comes out clean.
In medium saucepan, combine 1/4 cup butter, 3 tablespoons unsweetened cocoa powder, and 3 tablespoons buttermilk. Bring to boiling; remove from heat. Add powdered sugar and 1/2 teaspoon vanilla. Beat til smooth. Pour warm frosting over warm brownies, spreading evenly. Cool in pan on wire rack, then cut into bars.