Preheat oven to 375°.
To prepare the cake, coat a 15 x 10-inch jelly-roll pan with cooking spray, and dust with 2 teaspoons flour. Set prepared pan aside.
Lightly spoon flour into dry measuring cups; level with a knife. Combine 2 cups flour and next 4 ingredients (2 cups flour through salt) in a large bowl; stir well with a whisk. Combine water, 1/2 cup butter, and 1/4 cup cocoa in a small saucepan; bring to a boil, stirring frequently. Remove from heat; pour into flour mixture. Beat at medium speed of a mixer until well-blended. Add buttermilk, 1 teaspoon vanilla, and eggs; beat well. Pour batter into prepared pan; bake at 375° for 17 minutes or until a wooden pick inserted in center comes out clean. Place on a wire rack.
To prepare the icing, combine 6 tablespoons butter, milk, and 1/4 cup cocoa in a medium saucepan; bring to a boil, stirring constantly. Remove from heat, and gradually stir in powdered sugar, pecans, and 2 teaspoons vanilla. Spread over hot cake. Cool completely on wire rack.
Note: You can also make this recipe in a 13 x 9-inch baking pan. Bake at 375º for 22 minutes.