Melt chocolate together with butter in the top of a double saucepan over low heat. (The bottom saucepan only needs to be 1/3 full of water). Stand top saucepan to one side to cool.
In a large bowl, beat sugar and egg yolks until pale and light. Fold in cooled chocolate mix. Fold in the sifted flour.
Beat egg whites until stiff and shiny (do not overbeat) and fold into chocolate mixture.
Spray a 23cm x 6cm deep springform tin with cooking spray (or if you prefer, grease well with butter). Pour mixture into tin and bake at 200C for 25 minutes.
Carefully remove tin from the oven, pressing sides of the tin in to prevent the cake from cracking. Cake will wobble in the centre - don't worry, it is cooked!!!
Stand tin on the kitchen bench for 1/2 hour, and then place into the fridge. The "Wobbly" middle will set into a fudge texture as the cake cools.
To serve: Dust with icing sugar. Serve small slices with fresh seasonal berries and a light tangy ice-cream. I find this cake tastes best a couple of days after being made, and will store in the fridge for up to a week.
