Preheat oven to 350 degrees. Line 9 by 13 inch baking pan with aluminum foil. Extend end of foil 1 inch over sides of pan, grease and flour foil, and set aside. In a medium saucepan, melt 1 cup butter. Stir in 1 1/2 cups sugar and 1/2 cups cocoa. Remove pan from heat and beat in eggs, one at a time, until blended. Stir in flour and pecans until well blended. Pour into prepared pan and bake 25 minutes or until cake tester inserted into center comes out clean. Cool in pan on wire rack. Frosting: in a medium bowl, combine confectioner's sugar and cocoa powder. Beat in butter and vanilla until smooth. Beat in enough milk until fluffy and spreadable. Using ends of foil, lift brownie layer from pan. Spread top with frosting. Sprinkle with 1/2 cup pecans. Cut into 2 by 1 inch bars and store in air tight container.