Prepare cake according to directions on package but make 1 to 2 days ahead of time. Bake in oblong pan. When cool, poke holes in cake and pour 1/2 to 3/4 cup of Kahlua into holes and spread on top. Cover and cool 1 to 2 days.
When ready to prepare, cut cake in half and cut into cubes. Layer the cubes on bottom of a large deep glass bowl. Spread 2 boxes of prepared mousse, then cover with 1 container of Cool Whip. Repeat layers and when finished, sprinkle with chocolate shavings.