Preheat oven to 350°.
Prepare brownie mix.
When the brownies are slightly cooled, poke holes in the top with a fork; pour Kahlua over the top.
When the brownies have thoroughly soaked in the Kahlua, crumble into pieces; return to pan.
Prepare mousse mix according to package directions.
Assemble in layers, smoothly spread 1/2 chocolate mousse over crumbled brownies, then smoothly spread 1/2 whipped topping over mousse.
Sprinkle 1/2 crushed candy bars over whipped topping, repeat layers, and top with pecans.