Preheat the oven at 180 degree C and prepare two nos 7 inch cake tins with parchment paper at the bottom, while greasing the sides with butter and dusting with flour.
Cut the chocolate into small pieces and place them with butter on a bowl sitting over a pot of boiling water. The bowl should be at least a cm away from the water. Using a spatula, stir the butter and chocolate till the butter has melted and the chocolate starts to melt. Take it off the heat and let the chocolate melt in the residual heat.
Separate the yolk from the whites. Place all the yolks in a bowl and the sugar. Whisk for a few minutes until light and pale. By this time the chocolate mixture would have cooled down a bit.
Now slowly pour in the chocolate mixture into the egg yolks from one side. Beat the chocolate completely into the egg yolk mixture.
In a separate bowl, whisk the whites with a pinch of salt. I have used an electric hand mixer to do it. You can use your hand whisks or a stand mixer or whatever you are comfortable with.
You need to whisk the whites till they form a stiff peak. Then fold the egg white mixture into the chocolate mixture.
Divide this mousse into the two prepared tins. Bake in the oven for approximately 30 minutes or until the skewer comes out clean.
One thing to be noted here is that this is not a sponge cake and needs to be moist in the centre. Plus it is also flourless. So even a minute extra will dry out the cake. You need to watch it like a hawk towards the end.
Let the cakes cool down completely before you take them out of the tin.