Prepare and bake cake according to package directions to make two round layers. Cool 10 minutes in pans on a wire rack, then remove layers and cool completely. Reserve 1 layer for another use and cut other layer with a long, sharp knife into 1-inch cubes.
Prepare mousse mix according to package directions. Beat cream to stiff peaks.
Put half the cake cubes in the trifle bowl. Drizzle half the liqueur on top. Spread half the mousse, then half the whipped cream on top. Sprinkle half the chocolate. Repeat layering with remaining ingredients. Cover bowl with plastic wrap and chill for at least 4 hours before serving. Store leftovers in fridge.
Notes
*Prepare the cake, freeze and cut into cubes while still semi-frozen*Chill chocolate bars and pound them using a wooden spoon or rolling pin*Use any liqueur.....Bailey's, Kahlua, Gran Marnier