Make the cake according to the directions on the box for a 9x13" pan. As soon as you take the cake from the oven, prick all over the top of the cake with a fork. Immediately pour the sweetened condensed milk over the cake. Allow to cool completely. When cooled, frost with the Cool Whip and sprinkle the brickle bits over the top. Place in the refrigerator over night to allow the flavors to mix.