Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Grease and line a 7cm (2 ¾ inch) deep, 23cm (9 inch) round cake tin.
Break the White and Dark Chocolate into pieces and place in separate heatproof bowls. Put the bowls over saucepans of barely simmering water and allow the chocolate to melt. Remove the bowl from the pan of water and set aside whilst preparing the cake mixtures.
For the white chocolate cake, put half of the butter and sugar in a bowl and beat together until pale and creamy then beat in 2 eggs and the Madagascan Vanilla Extract. Sift 125g (4 ½ oz) flour and half of the Baking Powder on top. Add the melted White Chocolate and carefully mix the ingredients together. Leave aside.
For the dark chocolate cake, put the remaining butter and sugar in a bowl and mix as above, then beat in the remaining eggs. Sift the remaining flour, Baking Powder and the Cocoa on top. Add the melted Dark Chocolate and carefully mix the ingredients together. Leave aside.
Spoon a large spoonful of the white chocolate cake mix into the middle of the cake tin, and then a similar sized spoonful of dark chocolate cake mix on top. Tap the tin gently on the work surface to allow the mixtures to spread a little.
Continue spooning alternative spoonfuls of cake mix directly on top of each other, tapping occasionally to make sure the mixture spreads evenly. You should aim to spoon at least 6 quantities of each mixture on top of each other. You will see the rings of mixture form in the tin, the more mixture you spoon in.
Put the tin on a baking tray and bake in the middle of the oven for about 1 hour until firm and slightly risen – a skewer inserted into the centre of the cake should come out clean. Cool for 10 minutes before removing from the tin and transferring to a wire rack to cool completely.
Note: Take care with the spooning of the mixtures into the tin as it is this process that forms the stripes in the finished cake.