Heat oven to 350. Spray 2 lg baking sheets w/ cooking spray. In lg bowl, combine cake mix, eggs, oil, and cocoa powder. mix well. Drop by level heaping tbsp onto baking sheets, 2" apart. Bake 10 min. Let cool completely. In another bowl, beat frosting and whip topping until smooth. Spread a heaping tbsp of filling onto flat side of 1 cookie, top w/ another cookie, flat-side down. Repeat w/ remaining cookies.