Preheat oven to 350° F.
Spray 13x9-inch pan with cooking spray. For easy removal, coat pan with flour; set aside.
Stir cake mix and 1/2 cup cocoa in large mixing bowl. Add eggs, water and dressing; beat with mixer until well blended. Pour into prepared pan.
Bake 30 to 35 min. or until toothpick inserted into center comes out clean.
For frosting, beat cream cheese, milk and vanilla with mixer until well blended. Gradually add combined powdered sugar and remaining cocoa, beating well after each addition. Spread over cake.