Butter a 22 cm round non-stick cake (spring-form) tin. Preheat the oven to 140 C (fan). Put the chocolate pieces into a pan, add the butter, coffee granules and 125 ml water. Warm over a low heat until everything is melted. In the meantime, mix the flour, bicarbonate of soda and sugars in a bowl, first rubbing with your hands, then sieving to remove any lumps. Add the cocoa powder. Beat the eggs with the buttermilk and add to the mixture. Combine this with the chocolate sauce, until everything is well blended and smooth, then fold in the frozen raspberries. Pour into the baking tin and bake for ca. 1 hour 35 minutes. Take out and leave to cool on a wire rack.
When ready to decorate, prepare the ganache: chop the chocolate, heat the double cream and the sugar over a medium heat and pour the cream over the chocolate. Stir with a spoon until the chocolate has melted and the mixture is smooth.
When it has cooled down, cut the cake in half horizontally and sandwich together with some of the ganache, then pour the remaining ganache on top of the cake and spoon it around to cover everything evenly. If using, place the chocolate curls or shavings on top and around the cake. Leave to cool and set in the fridge, for at least one hour.
If you want to make chocolate curls, melt some chocolate in a bain-marie or a microwave and spread onto an even surface. Marble seems to work best, I used a smooth lacquered wooden chopping board. When the chocolate is spread in a thin and even layer, transfer the board to the fridge and leave to set. In order make curls, the chocolate must not be too soft nor too hard, so it's a bit of trial and error.
Take a big chopping knife and apply at a 45 degree angle, using a circular motion, until little cones form. Leave to cool in the fridge until ready to decorate the cake.