Heat oven to 350°F. Spray 13x9-inch pan with cooking spray. Coat with flour; set aside. Stir cake mix and ½ 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. Cool completely on wire rack. 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. Size Wise: Looking for a special treat? One slice of this heavenly cake provides a lot of chocolately flavor. How To Make A Triple Layer Cake: Grease and flour 3 (9-inch) round cake pans. Line bottoms of pans with waxed paper. Prepare cake batter as directed; pour evenly into prepared pans. Bake at 350°F 20 to 23 min. or until toothpick inserted in center comes out clean. Double frosting recipe. Fill and frost cake layers with prepared frosting.