Heat oven to 350 degrees. Pam spray a 13 x 9 x 2 inch baking pan. In a large bowl, beat butter or margarine and sugar until smooth, about two minutes. Beat in eggs and cocoa powder. On low speed, beat in flour, baking soda, water and milk until combined. Beat for one minute on medium high speed. Pour into prepared pan and bake at 350 degrees for 40 minutes or until a toothpick inserted into the center comes out clean. Remove from oven; cool on wire rack.
In a medium size bowl, beat together butter and cocoa powder.