Start by preheating your oven to (190°C / 375°F / gas mark 5) and lining an 8 inch loose bottom or springform cake pan.
Chop up the chocolate and butter and place into a heat proof (or microwave safe) bowl and melt over and bain marie (double boiler) or in the microwave, for 30 seconds increments, until fully melted, smooth and glossy and place to the side.
Separate the eggs, placing the whites into one bowl and the yolks into another.
Whisk the egg yolks, with the caster sugar, salt, instant coffee powder and vanilla extract, until light, pale and fluffy, and doubled in size. This should take around 5 minutes.
Next whip up the egg whites to stiff peaks, they should hold their shape and stand up straight on a rubber spatula without falling. Set this aside.
Now, add the melted chocolate mixture into the egg yolk mixture and whisk together.
Sift the cocoa powder into the egg yolk chocolate mixture and fold well, till fully combined.
Now, add one large rubber spatula (or spoonful) of egg white to the chocolate mixture and mix well. (This will loosen up the chocolate mixture)
Then, pour the chocolate mixture into the egg whites and fold together gently, until no egg white streaks remain. Being very careful not to knock the air out of the egg whites!
Pour the cake batter into the lined 8 inch cake pan and use the back of a rubber spatula to smooth it out.
Bake the cake for 25 minutes, until a crust has formed, it is no longer jiggly in the middle and the sides of the cake have started to come away from the pan.
Allow the cake to cool completely before removing from the pan and dust with cocoa powder.
Slice and serve with sweetened whipped cream and shaved chocolate.
Enjoy!