Preheat oven to 180C. Line an 20cm pan with aluminium foil. Make sure to extend the foil beyond the two opposite ends of the pan. This will help in lifting brownies out of pan. Lightly butter bottom and sides of foil-lined pan.
Cut butter into pieces and melt in a saucepan over low heat. Remove pan from heat and add unsweetened chocolate. Let stand 1 minute, then stir until smooth. Let cool for 10 minutes.
Whisk in 1/2 cup white sugar, 1 egg (make sure egg is at room temperature) and vanilla. Using a wooden spoon, stir in flour, bicarb soda and salt just until smooth. Spread dough evenly into pan.
Bake 10-12 minutes until toothpick inserted in center comes out with a moist crumb. Do not overbake. Cool completely on wire rack.
To make mousse: melt dark chocolate with the coffee in top of a double boiler over hot water. Stir often until smooth. Whisk in 3 egg yolks, one at a time. Remove from heat.
In a chilled medium bowl, beat 3/4 cup heavy cream just until soft peaks form. In another medium bowl, beat 3 egg whites just until soft peaks begin to form. Stir 1/4 of these beaten egg whites into the chocolate/coffee mixture, then carefully fold in the rest.
Gently fold in the whipped cream. Spread chocolate mousse evenly over the top of the cooled brownie in the pan.
To make topping: in a chilled medium sized bowl, beat 1/3 cup heavy cream and 1 tablespoon icing sugar until soft peaks form. Remove brownies from baking pan by grasping the overhanging foil. Cut into squares and transfer to serving dish. Place some whipped cream on top of each brownie and store in refrigerator.