Preheat oven to temperature indicated on devil's food cake mix package.

Prepare devil's food cake according to package directions in a jelly roll pan or 9 x 13-inch pan.

Let the cake cool completely.

Cut the cake into 1-inch cubes.

Use a meat tenderizing mallet to smash candy bars into small pieces (but not down to powder).

Prepare chocolate mousse according to package directions.

In a large transparent glass bowl or trifle dish, create the first layer with cake cubes.

Spread a layer of chocolate mousse over the cake cubes.

Sprinkle a layer of smashed toffee pieces over the mousse.

Spread a layer of Cool Whip over the toffee pieces.

Repeat the layering process 2 to 3 times.

Sprinkle the top layer of Cool Whip with toffee pieces.

Refrigerate for at least 30 minutes before serving.