Preheat oven to 350 degrees F. Grease and flour 10 inch tube pan. Prepare, bake, and cool cake following package directions. Split cake horizontally into 3 layers; set aside.
Chop 11 candy bars into pea-size pieces (see notes). Whip cream until stiff peaks form. Fold candy pieces into whipped cream.
To assemble, place one split cake layer on serving plate. Spread 1 1/2 cups whipped cream mixture on top. Repeat with remaining layers and shipped cream mixture. Frost sides and top with remaining filling. Chop remaining candy bar coarsely. Sprinkle over top. Refrigerate until ready to serve.