In large bowl, prepare cake batter according to package directions; divide batter evenly between prepared pans. Bake 25 to 30 minutes or until toothpick inserted in center comes out clean. Cool in pan on wire rack 10 minutes; remove from pan and cool completely. In large microwave-safe bowl, melt 6 ounces (6 squares) chopped chocolate. Quickly fold in about half of the whipped topping to lighten chocolate. Fold in remaining whipped topping; mix until well blended. Spoon mousse into cavities of cooled cakes. Assemble cake by inverting top layer onto bottom layer. Ice with remaining mousse. Chill cake while preparing ganache glaze.