Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Lightly grease a 24cm (9 ¾ inch) ring cake tin.

Break 100g (3 ½ oz) Extra Dark Chocolate into pieces and place in a small heatproof bowl over a pan of gently simmering water. Leave to melt, then remove from the water and set aside for 5 minutes.

Meanwhile, sieve the flour and Baking Powder into a bowl and stir in the sugar. Make a well in the centre. Separately, Mix the egg yolks and oil with 100ml (3 ½ fl.oz) cold water. Then in another bowl, whisk the egg whites with the Cream of Tartar until stiff.

Pour the egg yolk and oil mixture into the well and add the warm chocolate. Mix to form a smooth batter. Carefully fold the egg whites into the chocolate batter until thoroughly mixed.

Pour into the prepared cake tin and bake for about 50 minutes until risen and firm to the touch, and a skewer inserted into the sponge comes out clean. Cool for 5 minutes before turning on to a wire rack to cool completely.

For the decoration, using a vegetable peeler, shave some of the Extra Dark Chocolate into small curls to make about 30ml (2 tbsp) shavings and set aside. Break the remaining Chocolate into pieces and place in a small heatproof bowl with the butter and 20ml (4 tsp) cold water. Put the bowl over a pan of barely simmering water until melted then remove from the water and mix well until smooth and glossy.

Put a board under the cake rack and spoon a little chocolate glaze over the cake, allowing it to drip down the sides. Sprinkle with the chocolate shavings and leave for a few minutes to set before slicing to serve with the remaining glaze to spoon over.