Bake cake according as directed on box in a 9x13" pan.
As soon as you take it form the oven, prick all over the top and pour the Kahlua over the top and let it soak in.
Make the chocolate mousse. Break the cake into small pieces and put about half of it into large bowl. Layer half of the mousse, half the cool whip and half the candy bars. Repeat with another layer using the other half of ingredients.
Chill in fridge and serve. This serves 18. It can easily be halved.
I put mine in a pretty crystal bowl, it looks really nice.