Preheat the oven to 140°C fan / 320°F / Gas 3.Grease and line a 23cm (9inch) springform pan with butter and baking/parchment paper. In a medium-sized heatproof bowl, melt the butter and chocolate in the microwave for a few seconds or over a pan of gently simmering water (double boiler method), stirring occasionally. Once smooth, glossy and completely melted set aside to cool slightly.
Separate the egg yolks and whites between two large mixing bowls. Whisk the egg whites for 2 to 3 minutes until soft peaks form.
Add the sugar and vanilla to the yolks and beat until pale and the sugar has dissolved. Approxiamately 5-6 minutes.
Sift the cocoa powder into the egg yolk mixture with a pinch of salt and gently stir to combine.
Add the melted chocolate mixture to the egg yolks and stir until well combined.
Add 2 dessert spoons of egg whites into the chocolate mix and gently stir through, then gently fold in the remaining egg whites. Be careful not to over mix or all that precious air you whisked into them will be lost.
Pour the batter into the prepared baking pan.
Baking: Place it on the medium shelf of the oven and bake for approximately an hour or until an inserted skewer comes out clean.
Allow the cake to cool for 10 minutes in the pan before removing the outer ring, leaving it to cool on a wire rack. I like to leave the base of the pan on until it is completely cooled, as it is quite a crumbly cake and can easily break.Serve with a dusting of cocoa powder or icing sugar and fresh berries.