Preheat the oven to 160°C (140°C Fan, Gas Mark 3). Lightly grease a deep 10cm (4 inch) diameter cake tin.

Break the Dark Chocolate into pieces and place in a heatproof bowl over a saucepan of gently simmering water. Cut the butter into small cubes and add to the Chocolate. Leave to melt, then remove from the water and set aside to cool for 10 minutes.

Meanwhile, whisk the egg white until thick and foamy. In another bowl, whisk the egg yolk and sugar together until thick, pale and creamy.

Stir the melted Chocolate mixture into the whisked egg yolk and sugar. Sift the flour and Baking Powder on top, add the ground hazelnuts and whisked egg white. Carefully fold all the ingredients together until well mixed, taking care not to beat too much air from the mixture.

Transfer to the prepared cake tin, smooth the top and bake in the centre of the oven for about 40 minutes until just firm to the touch. Allow to cool in the tin for 30 minutes on a wire rack before removing from the tin and placing back on the wire rack to cool completely. Stand the rack over a board.

To decorate, break off 25g (1oz) Dark Chocolate, grate and set aside. Break up a further 25g (1oz) Dark Chocolate into a heatproof bowl and melt as above. Remove from the water and spread thickly in a strip over a small board. Leave in a cool place to set.

Meanwhile, break up the remaining Chocolate into pieces and place in a heatproof bowl. Add the butter and 15ml (1 tbsp) water. Melt as above. Remove from the water and set aside for about 20 minutes to cool to a thick, coating consistency.

When the Chocolate starts to thicken, spoon half the ganache on the top of the cake. Tap the rack on the board to allow the icing to smooth on top. Use a small bladed palette knife to spread the remaining ganache round the sides of the cake. Carefully press the grated Dark Chocolate on to the side of the cake. Leave to stand in a cool place for several minutes to set.

For a final finishing touch, use a butter curler or teaspoon to drag through the spread chocolate to make a curl. Place in the centre of the cake to serve.