Line two 9-inch square baking pans with parchment paper. Spray bottoms with non-stick cooking spray. Set aside.
Prepare brownies batter according to recipe or instructions on the package, using the 2 egg-recipe and substituting milk for the water.
Pour equal amounts of batter into each prepared pan.
Cook brownies according to recipe or package instructions.
Allow brownies to cool completely.
Set one brownie pan aside.
Invert other pan and remove brownie in one piece. Return to pan upside down. Remove parchment paper.
Brush the brownie evenly with 1/2 of Kahlua.
Then top with 1/2 of the mousse, spreading it evenly over the brownie.
Spread 1/2 of the whipped cream over the mousse.
Remove second brownie from pan in the same fashion as the first.
Brush with remaining Kahlua.
Place it on top of first brownie.
Spread remaining mousse and whipped cream evenly over the top in the same manner as the first brownie.
Drizzle with hot fudge sauce.
Refrigerate, uncovered if less than 2 hours, otherwise covered or in a sealed container, until ready to serve.