Preheat oven according to brownie package directions. Bake brownies according to package directions; let cool. Prick holes in the top of the brownies with a fork and pour the coffee liqueur over brownies; set aside.
Prepare chocolate mousse according to package directions. Break candy bars into small pieces in food processor or by gently tapping the wrapped bars with a hammer. Break up 1/2 the brownies into small pieces and place in the bottom of a large glass bowl. Cover with 1/2 the mousse, then 1/2 the candy, and then 1/2 the whipped topping. Repeat layers with remaining ingredients.
Note: Instead of the coffee liqueur, substitute a mixture of 1 teaspoon sugar and 4 tablespoons black coffee. Or just leave out the coffee flavoring entirely. This is absolutely delicious and it is as it says Death By Chocolate.
