Preheat the oven to 180C/350F.

Line the bottom of a 23cm springform cake tin with baking parchment.

Melt the chocolate either in a double boiler or a microwave, then let the butter melt in the warm chocolate.

Beat 2 whole eggs and 4 egg yolks with 75g of the caster sugar, then gently add the chocolate mixture.

In another bowl, whisk the 4 egg whites until foamy, then gradually add the 100g of sugar and whisk until the whites are holding their shape, but not too stiff.

Lighten the chocolate mixture with a dollop of egg whites, and then fold in the rest of the whites.

Pour into the prepared tin and bake for 40-45 minutes or until the cake is risen and cracked and the centre is no longer wobbly.

Cool the cake in its tin on a wire rack; the middle will sink as it cools.

Just prior to serving, place the still tin-bound cake on a cake stand or plate for serving and carefully remove the cake from its tin.

Whip the cream until it is soft and add the vanilla until the cream is firm but not stiff.

Fill the centre crater of the cake with the whipped cream, easing it out gently towards the edges of the cake and sprinkle grated chocolate on top for decoration.