To make the marble shards, line a 30 x 20 cm baking tray with greaseproof paper. Break 150g of 72% Extra Dark Chocolate in a heatproof bowl. Place the bowl over a pan of barely simmering water and stir until melted. Also melt 75g of White Chocolate as above.
Pour the melted 72% Extra Dark Chocolate into the lined baking tray. Spread the chocolate to the edges of the tray with a pallet knife and tap the tray on your surface to smooth. Spoon blobs of the White Chocolate randomly over the 72% Extra Dark Chocolate and using a cocktail stick or wooden skewer create a marble effect by swirling the two chocolates in a figure of eight pattern repeatedly. Sprinkle the White Chocolate Chunks over the melted chocolate.
Place the chocolate in the fridge and allow to chill for 3 hours or until completely set. Once set break the chocolate into shards by hand.
Whilst the marble shards are setting make the ganache. Put the cream in a saucepan and heat gently over a low heat, until the cream is bubbling. Keep stirring whilst the cream is heating. Once boiling, remove the cream from the heat and break the Milk Chocolate into squares. Add the Milk Chocolate to the cream and stir until melted. Chill in the fridge for 2 hours until set and thickened.
Meanwhile make the sponge cake, preheat the oven to 180ᵒc (160ᵒc fan oven, 350ᵒF, gas 4). Grease and line 4 x 7inch round cake tins. Do not use loose bottomed tins as the cake batter is very liquid. Put all the ingredients for the cake except the hot water into a bowl and fold carefully until all the ingredients are mixed.
Add the boiling water and with a hand whisk, mix until you have a smooth consistency. The cake batter will have a very liquid consistency. Pour the mixture into a jug and divide the mixture evenly between the cake tins. Bake for 30 minutes until cooked and a cocktail stick or skewer comes out clean. Leave to cool.
Once the cake is cooled, use a small amount of ganache to stick the base tier of the cake to the cake board. Spread the ganache on each layer of cake using a pallet knife and stack the cake together. Then spread a thin amount of ganache on the outside of your cake, starting from the base and working up the cake. Chill for around 30 minutes in the fridge until set.
Add another layer of ganache around the whole cake and once covered, use a hot palette knife (you can dip this in a cup of boiling water) to get a smooth finish.
To make the chocolate drip, break 150g of 72% Extra Dark Chocolate in a heatproof bowl. Place the bowl over a pan of barely simmering water and stir until melted. Leave to cool slightly for around 5 minutes. Pour the melted chocolate onto the top of your cake and using a palette knife, spread over the top of the cake and push to the edges of the cake so that it starts to drip over the edge.
Whilst the chocolate drips are still setting, insert your shards, by making some incisions in the top of the cake (around 1inch depth) using a small sharp knife to help the shards stay upright.