Day before serving: Line 13 x 9 baking pan with foil. Grease with butter. Flour pan and shake out excess. Prepare and bake cake according to package directions and pour into lined pan. When cook, remove about 1/3 of cake for other use. Lift foil to cover, cut end. Combine Kahlua and coffee. Pierce cake all over with fork. Don't damage foil. Pour liquid evenly over cake. Cover with plastic wrap and refrigerate overnight or at least several hours before assembling.
Assembly: Prepare mousse according to package directions. Refrigerate 1 to 1 1/2 hours to thicken. (Not longer or will be too thick.) In trifle dish (preferably glass so you can see the layers), put layer of large chunks of cake (about 1/2 of cake). Top with 1/2 of the mousse. Top with 1/2 of the broken Skor bars. Top with 1 of the containers of Cool Whip. Repeat: cake, mousse, candy, Cool Whip. Garnish with chocolate shavings from candy bar.
