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Grease and bottom line a 24cm spring form cake tin. Heat oven to 160C/fan 140C/gas. Melt 120g chocolate in a bowl. Cool for 5 mins, then beat in egg yolks. In a bowl, beat the butter and 100g sugar until soft and creamy. Mix in the melted chocolate and yolks.
Whisk the egg whites to firm peaks then beat in remaining 120g sugar in three batches and continue whisking until stiff. Beat a third of the meringue into the chocolate mix. Then, gently fold in alternate spoonfuls of the flour and remaining meringue.
Spoon into the tin, level and bake 40-45 mins until the top forms a light crust. Test the centre with a metal skewer, it should come out clean. Remove, cool 10 mins, before turning out of the tin. When cooled, cut in half carefully as it is fragile, with a large serrated knife.
To make the filling: in a bowl beat the double cream until thickened, but not stiff. Then mix in the crème fraîche and the zest.
Spread one half of the cake with all the zesty cream and sandwich together with the other half. Place on a rack with a large plate beneath.
To make the ganache icing: break up the chocolate into a pan and add the double cream. Melt over the lowest heat and stir until smooth, then cool until the consistency of thick pouring cream. Pour over the cake.
Let excess icing run onto the plate and spoon it back over to fill any gaps. Spread the icing evenly on the top and around the sides using the back of a spoon or a palette knife. While the icing sets sprinkle the grated chocolate on top and around the sides.
Make the curls: You will need a large block of chocolate. Ensure the chocolate is at warm room temperature and placed on a worktop. Gordon used a sharp knife to make the curls, but it’s easier to draw a swivel vegetable peeler down the side of a thick block of chocolate.
Place a large ring cutter (about 10cm) in the centre of the cake and fill with the chocolate curls. Carefully remove the ring and let the curls cascade gently out. Serve at room temperature. If chilled ahead the cake should be allowed to come to room temperature before serving.
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