Line 9 by 13 inch baking pan with aluminum foil. Extend foil 1 inch over sides of pan. Lightly 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 powder. 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 at 350°F for 25 to 30 minutes or until cake tester inserted into center comes out clean. Cool in pan on wire rack.
Frosting: In a medium bowl, combine confectioners sugar and 2 tablespoons cocoa powder. Beat in 1/2 cup softened butter and vanilla until smooth. Beat in enough milk until frosting is fluffy and spreadable.
Using ends of foil, lift brownie layer from pan. Spread top with frosting. Cut into 2 by 1 inch bars and store in air tight container.