Preheat oven to 375°.
Coat a 13 x 9-inch pan with cooking spray, and dust with 2 teaspoons flour. Set aside.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine 2 cups flour and next 4 ingredients (through salt) in a large bowl, stirring well with a whisk. Combine 3/4 cup water, 1/2 cup butter, and 1/4 cup cocoa in a small saucepan; bring to a boil, stirring frequently. Add to flour mixture. Beat at medium speed with an electric mixer until well blended. Add buttermilk, 1 teaspoon vanilla, and eggs; beat well. Pour batter into the prepared pan. Bake at 375° for 22 minutes or until a wooden pick inserted in center comes out clean. Place on a wire rack.
Combine 6 tablespoons butter, fat-free milk, and remaining 1/4 cup cocoa in a saucepan; bring to a boil, stirring constantly. Remove from heat. Gradually stir in powdered sugar and remaining 2 teaspoons vanilla. Spread over hot cake. Sprinkle cake with pecans. Cool completely on wire rack.