Start with the salted caramel. You can make this up to 2 days in advance. Place the sugar in a large saucepan with 50ml of water, and gently heat over a low heat, swirling the pan but not stirring, until just melted.
Turn up the heat a little and simmer gently, swirling regularly, until you have a clear, very dark golden caramel.
Cube the butter. Remove the pan from the heat, then carefully and quickly whisk in the butter and cream. It will splutter a bit, but don’t worry about that. Keep whisking and beating until smooth, then beat in the ½ teaspoon of sea salt.
Let it cool. If making the caramel in advance, keep it chilled but bring back to room temperature before using.
For the cake, preheat the oven to 150ºC/gas 2. Grease and line the bases of two 23cm cake tins.
Break the chocolate into a saucepan, cube and add the butter, along with the coffee and a pinch of fine salt. Melt over a low heat until combined.
In a large mixing bowl, mix the flour, sugars and cocoa until well combined.
Beat the buttermilk into the melted chocolate mixture, then beat and mix in the eggs.
Pour this mixture into the flour-sugar mixture and mix well.
Spoon into the cake tins and bake for around 1 hour 15 minutes, or until cooked through and a skewer inserted in the centre comes out clean.
Cool the cakes in the tins for 10 minutes, then turn onto a wire rack to cool completely.
Meanwhile, make the ganache. Break the chocolate into a bowl.
Pour the double cream into a small pan and gently heat. Just before it starts bubbling, remove the cream from the heat and pour it over the chocolate (never add the chocolate to the cream or it will seize up).
Leave the ganache to sit for a minute, then stir with a wooden spoon until completely combined.
While the ganache is still slightly warm, evenly spoon one-third of it over one of the sponges and spread to the edge. Leave the rest of the ganache to cool completely at room temperature.
Place the unfrosted cake on a board or plate and drizzle with a few tablespoons of the salted caramel (you may need to warm it slightly before drizzling).
Using an electric mixer or whisk, beat the cooled ganache for 3 to 5 minutes, until light and fluffy. Spread over the salted caramel, top with the other sponge and finish with white chocolate shavings, if you like.