Yum! It was so good. The caramel icing was quite difficult to work with, because it was either setting too quickly and not spreading or spreading too easily and dripping down the sides. I decided to just leave it to set in the fridge and then tidied it up afterwards by using a spatula dipped in boiling water.
- Preheat the oven to 180 C.
- Sift the flour, salt and sugar into a bowl of a large free-standing electric mixer (or into a mixing bowl if you are going to use a hand held electric mixer) Add the butter to the bowl, in pieces. Whisk the eggs with the buttermilk and vanilla using a fork, then add to the bowl.Beat on low speed until everything is thoroughly combined and very smooth, thick and light.
- Divide the mixture evenly among the 3 tins and spread level. Bake for about 25 minutes until the sponges are springy when gently pressed in the centre and starting to shrink away from the sides of the tins.
- Set the tins on a wire rack and run a round-bladed knife around the insides to loosen the sponges, then cool for 2 minutes before turning out. Leave to cool completely.
- Meanwhile, make the caramel filling. Put 175g of the weighed butter into a medium-sized pan with the muscovado sugar and cream. Heat gently until the butter has melted, then bring to the boil, stirring. Reduce the heat and simmer gently for 5 minutes, stirring frequently. Pour the mixture into a heatproof mixing bowl and beat in the icing sugar, using an electric mixer. When the sugar has been incorporated, continue beating until the mixture is fluffy and barely warm. Gradually beat in the rest of the butter and the salt. Add more salt, if needed.
- Put the pieces of dark chocolate into a heatproof bowl and set over a pan of steaming hot (not boiling) water and melt gently, stirring until smooth. Spoon about half the chocolate into another bowl, and stir in slightly less than 1/4 of the caramel mixture.
- To assemble the cake, set one sponge layer on a serving plate and spread over one-third of the caramel mixture. Spread a second sponge layer with the chocolate-caramel mixture and set on the first layer. Place the third sponge layer on top. Leave the cake to set in the fridge slightly (to avoid a landslide!) then cover the top and then the sides of the assembled cake with the remaining caramel mixture, working quickly before the mixture firms up.
- Spoon the remaining melted chocolate into the disposable piping bag and snip off the end. From the centre, pipe a spiral on the cake, then 'feather' the surface by drawing a cocktail stick through the icing from the centre to the edge at regular intervals. Then, in between the feathered lines, draw the stick back to the centre from the edge. To marble the icing, pipe random loops and circles of chocolate onto the caramel icing, then swirl the two together with the cocktail stick.
- Leave to set. The cake can be kept, in an airtight container, for up to 4 days.