Prepare cake mix as directed on pkg. Divide cake mix into 2 square or round cake pans, and bake according to pkg. directions; let cool.
While cake is cooling, make the pudding according to pkg. directions.
Put one cake on bottom of big trifle bowl, add chocolate pudding, and then Cool Whip. Put the other cake on top of the Cool Whip. Add pudding and the other Cool Whip. Top with Hershey's chocolate bar in pieces.