To prepare cake, coat a 15 by 10-inch jelly roll pan with cooking spray.
Combine 2 cups flour and next 4 ingredients in a large bowl and stir well with a whisk. Combine water, 1/2 cup butter, and 1/4 cup cocoa in small saucepan. Bring to a boil, stirring frequently. Remove from heat and pour into flour mixture. Beat at medium speed until well blended. Add buttermilk, 1 teaspoon vanilla and eggs and beat well. Pour batter into prepared pan and bake at 375 degrees for 17 minutes or until a wooden pick comes out clean.
To prepare icing, combine 6 tablespoons butter, milk, and 1/4 cup cocoa in a mediium saucepan and 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. Serves 20.