Add third cake layer. Frost with remaining shipping mixture. If desired, cover and chill for up to 2 hours. Slice into wedges to serve. If desired, drizzle fudge sauce onto dessert plates before adding cake slices.
Mascarpone is a rich Italian cream cheese often served with fruit for dessert.
You can buy it at the cheese counter in your supermarket. While it is baking, mix together the chocolate syrup and the Kahlua. Pour about half of that mixture into the serving dish. Pour half of what is left of the syrup mixture on top of the brownie mix, and refrigerate. Make the Mousse according to package directions.