With long, serrated knife and cutting horizontally, carefully cut off top 1/2 inch of cake. Set top portion of cake aside.
Cut down into cake 1/2 inch from outer edge and 1/2 inch from hole, leaving a "wall" of cake 1/2 inch thick. Remove center with curved knife or spoon, being careful to leave base of cake at bottom 1/2 inch thick. Discard or reserve crumbs for a later use. Place cake on serving plate.
Spoon pie filling evenly into cavity of cake. Fold cocoa into whipped topping until well blended. Spoon 1/2 cup topping mixture over cut surface of cake. Replace top portion of cake.
Frost sides and top of cake with remaining whipped topping mixture. Sprinkle top with grated chocolate. Refrigerate at least 1 hour or until serving time. Store in refrigerator.