Preheat the oven to 180°C (160°C Fan, Gas Mark 4). Grease and line a deep 20cm (8 inch) round cake tin.
Break the Extra Dark Chocolate into pieces and place in a large heatproof bowl with the butter pieces. Put the bowl over a saucepan of barely simmering water and leave to melt. Remove from the water and mix in the sugar and vanilla extract. Leave aside for 10 minutes. Meanwhile, whisk the egg whites in a clean, grease-free bowl until stiff.
Beat the egg yolks into the mixture. Sift the flour, Baking Powder and cocoa on top and add the ground almonds. Add the egg whites on top then carefully fold all the ingredients together, retaining as much air as possible.
Place the mixture into the tin and smooth the top. Place 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, remove from the tin and transfer to a wire rack to cool completely
To decorate, break the Dark Chocolate into a small saucepan. Add the butter and pour in 50ml (2fl.oz) water. Heat gently, stirring occasionally, until melted together and smooth. Remove from the heat and set aside for about 20 minutes to cool and thicken.
Dust the work surface lightly with icing sugar and knead the Marzipan to soften it. Flatten slightly then add 1tsp cocoa powder. Knead to distribute the powder then gradually add the remaining cocoa powder and knead in.
Brush the top of the cake with apricot jam. Roll out the chocolate Marzipan to fit the top of the cake and lay it on top. Smooth and trim as necessary. Put the cake on a wire rack over a tray.
Once thickened, slowly pour the ganache over the top of the cake from the middle, in a thin stream. Use a small bladed palette knife to smooth over any gaps and leave to stand in a cool place for several minutes to allow the icing to set.
Carefully lift the cake on to a serving plate. For a firm finish, chill the cake for 30 minutes before serving.