Grease a 15x10x1-inch or a 13x9x2-inch baking pan; set aside. In a medium mixing bowl combine the flour, sugar, baking soda, cinnamon, and salt; set aside.
In a medium saucepan combine the 1 cup butter, water, and the 1/3 cup cocoa powder. Bring mixture just to boiling, stirring constantly. Remove from heat. Add the chocolate mixture to the flour mixture; beat with an electric mixer on medium speed until combined. Add the eggs, 1/2 cup buttermilk, and 1-1/2 teaspoons vanilla. Beat for 1 minute (batter will be thin). Pour batter into the prepared pan, spreading evenly.
Bake brownies in a 350 degree F oven about 25 minutes for the 15x10x1-inch pan, 35 minutes for the 13x9x2-inch pan, or until a wooden toothpick inserted in the center comes out clean.
Meanwhile, in a medium saucepan combine the 1/4 cup butter, the 3 tablespoons unsweetened cocoa powder, and the 3 tablespoons buttermilk. Bring mixture to boiling. Remove from heat. Add powdered sugar and 1/2 teaspoon vanilla. Beat until smooth. If desired, stir in pecans.
Pour warm frosting over the warm brownies, spreading evenly. Cool on a wire rack. Cut into bars. Makes 24 brownies.