Pre-heat the oven to 180ºC/350ºF. Grease a springform cake pan with soft butter then line with baking paper.
Beat the eggs and sugar in the bowl of a stand mixer until voluminous, pale and fluffy.
In the meantime, melt the chocolate and butter over a double boiler until smooth and glossy. Stir in the vanilla and allow to cool for a few minutes.
Pour the chocolate into the egg mixture, whilst continuously whipping, until combined.
Sift the cocoa powder and salt over the batter and gently fold in.
Pour the batter into the prepared tin and place in the oven. Bake for 20-25 minutes until the cake has risen, feels firm and a skewer inserted comes out with moist crumbs (a clean skewer is also an indication that the cake is cooked).
Remove from the oven and allow to cool completely in the cake pan. Once cool, remove and serve with thick cream and berries.