Place the chocolate and butter in a bowl and place the bowl on a double boiler on medium heat. dissolve the chocolate and butter until it resembles a chocolate sauce. Once done add the brewed coffee. Mix well and leave aside. (Double boiler: Heat half a pot of water on medium heat, place a bowl on top of the pot of water)
In a separate bowl add the egg yolks, sugar ( leave 10g aside for the egg whites) and vanilla and whisk until pale and creamy, Then place this on the double boiler and cook until its thick and creamy. Note that this is a crucial step and keeping it too long will make it scrambled eggs, therefore mix consistently
Place, the egg white and the rest of the white sugar and beat until firm peaks form. Leave it aside.
In a mixing bowl add cream and whisk until soft peaks are formed (Do not over whisk)
Mix the dissolved chocolate mixture into the egg yolks
Then Using a spatula fold in the egg white, egg yolk mixture & Whipped cream until fully incorporated ( Do not overwork the mixture, this will result in deflating, use about 10 folds to fully incorporate the mixture)
Pour the mixture into glasses or pots that you want to present the chocolate mousse, cover and refrigerate for at least 4 hours or overnight
Add some berries and whipped cream and serve chilled