Bake brownies according to directions. Let cool. Poke holes throughout brownies using a fork and then pour in Kahlúa. Set aside. Whip chocolate mousse mix according to package directions. Break up candy bars into small pieces. Break up half the brownies and place them in the bottom of a trifle dish. Next, cover brownies with half the candy bar pieces, followed by half the chocolate mousse. Layer half the whipped topping over the mousse. Continue layering with remaining ingredients, and garnish with tinned sour cherries and mint leaves if desired. Serve chilled.