Preheat oven to 350°F. Spray Tasty-Fill? pans with vegetable pan spray.
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.
In small microwave-safe bowl, heat the cream until hot, but not boiling (about 30 seconds). Add remaining 2 ounces (2 squares) chopped chocolate. Let stand 1 minute; stir until chocolate melts. Cool ganache about 15 minutes. Pour over top of cake and let drip over sides of cake. Refrigerate until ready to serve.