Prepare brownies according to package directions. Cool on wire rack.
Prepare mousse mix according to package directions, except do not chill after beating.
Using a toothpick, poke some holes into brownies. Brush with the liqueur, if you like. Coarsely crumble and place half of the brownies in a 3-1/2- to 5-quart glass serving bowl.
Spread half the mousse over brownies in bowl. Spread half the whipped topping over mousse. Sprinkle with half the toffee and half the nuts.
Repeat layers with remaining ingredients. Cover and chill in refrigerator for at least 4 hours or up to 24 hours. Makes 12 to 15 servings.