Sponge cakes: To a heavy based saucepan add the chocolate, sour cream, muscovado sugar and set over a low heat. Stir occasionally until it completely melts and then set aside.
Preheat the oven to 180°C fan / 400°F / Gas 6.
Line and grease 2 x 20cm springforms or cake pans and set aside until needed.
In a large bowl or stand mixer add the eggs and caster sugar, whisk until they are pale and double in volume.
Add the vanilla extract, water and the oil and whisk again to incorporate.
Sift the flour, cocoa powder, salt and bicarbonate of soda into the egg mixture and gently fold it in. Half way through folding, pour in the melted chocolate mixture and continue folding until fully incorporated.
Divide the batter between the two cake pans.
Baking: Place the cake pans onto a baking tray in the centre of your preheated oven and bake for 25-30 minutes, until a skewer or toothpick inserted into the centre comes out with a few cake crumbs on it. If it has wet batter leave to bake.
Once baked remove from the oven and leave to cool in the pans for 5 minutes, then remove from the pans and leave to cool completely on a wire rack.
Ganache: In a small saucepan bring the cream to the boil. In a medium bowl add the chopped dark and milk chocolate. Once the cream has come to the boil pour it over the chocolate and leave to sit for 3 minutes without stirring.
After the time has passed, whisk the cream and chocolate together until it becomes silky smooth and lump free. Leave until the ganache is thick and spreadable. In hot weather it may be necessary to place the ganache in the fridge but remember to keep an eye on it to prevent it from solidifying. Assembly: Place one of the cakes on a suitable serving plate and spread the top with about 1/2 cm of ganache. Place the second sponge on top and spread the remaining ganache over the top and sides until the cake is completely covered.
Keep in a cool place until ready to serve.