This is a decadent celebration cake to satisfy even the most ardent chocoholic.
Heat the oven to 180°C/160°C fan/350F/Gas 4. For the sponges, tip the caster sugar, dark muscovado sugar, chocolate, espresso powder, butter and 250ml of water into a saucepan and stir the mixture over a low heat until the sugar, chocolate and butter have melted and the mixture is smooth. Remove the pan from the heat and leave the mixture to cool for 5 minutes, then whisk in the buttermilk, then the egg and egg yolks until smooth and well combined.
Sift the flour, cocoa powder and bicarbonate of soda into a large bowl. Make a well in the centre, then pour in the chocolate mixture and whisk until just combined.
Divide the cake batter between the prepared tins, then bake for 30–35 minutes, until a skewer inserted into the centre of each sponge comes out clean.
Leave the sponges to cool for 10 minutes in the tins, then loosen the edges and turn them out onto a wire rack. Leave to cool completely.
While the sponges are cooling, make the ganache. Put the chopped chocolate in a heatproof bowl. Heat the cream in a pan over a medium heat to a gentle simmer (don’t let it boil), then immediately remove the pan from the heat. Pour the hot cream over the chopped chocolate and leave it to stand for 5 minutes. Gently stir the chocolate in small circles, starting at the centre of the bowl until the ganache comes together and is evenly coloured and glossy. Leave to cool until the ganache has thickened to the consistency of soft fudge.
For the chocolate curls, melt the chocolate in a bowl set over a pan of gently simmering water. Spread the chocolate in a thin layer over the base of the chilled baking tray. Leave it to set, then holding the cake scraper at a 30-degree angle push the set chocolate away from you to create large, decorative curls. Adjust the angle of the scraper to make the curls tighter or looser, as you wish.
To assemble the cake, whisk the cooled ganache with an electric hand whisk on high speed for 3–5 minutes until pale and very fluffy.
Place one of the sponges on a serving plate and spread it with about one third of the ganache. Place the second sponge on top to sandwich the filling and spread the remaining ganache over the top and side of the cake with a palette knife. Decorate with the chocolate curls piled on top.