About this recipe:The name of this cake, also known as American fudge cake, indicates just how unashamedly rich it is, and even more so if served as a dessert with chocolate ice cream or lashings of whipped cream.
Preheat the oven to 180 C / Gas 4. Grease two 23cm/9in sandwich tins with butter and line with baking paper. Put the chocolate and 400g butter in a large, heatproof bowl set over a saucepan of simmering water. Heat gently until smooth but not hot. Stir together and remove from the heat.
Using a wooden spoon, beat in the caster and brown sugar. Gradually stir in the flour. Beat the eggs and the vanilla extract in a separate bowl, then beat well into the chocolate mixture. Turn the mixture into the prepared tins and spread out evenly.
Bake the cakes for about 40 minutes, or until a skewer inserted in the centre comes out clean. Leave to cool in the tin.
Meanwhile, make the filling and topping. Put the cocoa powder in a small bowl, add the boiling water and stir to form a smooth paste. Leave to cool. Put the butter and vanilla in a large bowl, sift in the icing sugar and beat with a wooden spoon until smooth. Beat in the cooled cocoa mixture.
When the cakes are cold, remove from the tins and sandwich together with half the filling. Swirl the remaining mixture over the top of the cake, then top with chocolate curls.