Follow directions on the cake mix and bake as instructed in a 8x10 pan. Before you bake your cake set out your Cool Whip to become soft....
After the cake as cooled poke several holes in it using a fork. Then pour your Kahlua over the entire cake and let set for at least one hour (overnight is also okay).
After you have added the Kahlua to your cake start making the mousse. Follow the directions on the Dream Whip box (using all pkgs inside the box)this is where you also use the instant pudding mix. When it's done put in the refrigerator.
Take a large zip loc bag and place your candy bars (out of their wrappers) inside the bag and smash with a small hammer or the side of a meat tenderizer. I laid a thin dish towel over the bag to help with holes not being made. Remove your mousse from the refrigerator to become room temperature.
Take your cake and criss cross cut it into small squares. Do not worry about them staying perfect as it doesn't matter.
Take your trifle dish (I bought one relatively inexpensive) and start layering. Lay down a nice layer of cake chunks, then a layer of mousse, then your Cool Whip and a layer of crushed candy bar and repeat. Your might have enough to make a small one as well. The extra cake is delicious the way it is as my family loved it as well it was so moist.