1. Heat oven to 350 degrees F. Coat 4 baking sheets with nonstick cooking spray and set aside.
2. In a large bowl, combine cake mix, eggs, oil and cocoa powder. Beat on low for 30 seconds or until combined. Beat on medium speed for 2 more minutes; batter will be very thick. Drop batter by level tablespoons onto prepared baking sheets, 2 inches apart, for a total of 40 cookies. Bake at 350 degrees F for 10 minutes, rotating baking sheets halfway through. Use a metal spatula to quickly transfer cookies to racks and allow to cool.
3. Meanwhile, in a small bowl, stir together vanilla frosting and whipped topping. Spread a heaping tablespoon of frosting mixture onto flat side of one cooled cookie, then place the flat side of a second cookie on top. Repeat.