Butter a 20cm round cake tin (7.5cm deep) and line the base. Preheat the oven to fan 140C/conventional 160C/ gas 3. Break 200g good quality dark chocolate in pieces into a medium, heavy-based pan. Cut 200g butter into pieces and tip in with the chocolate, then mix 1 tbsp instant coffee granules into 125ml cold water and pour into the pan. Warm through over a low heat just until everything is melted – don’t overheat. Or melt in the microwave on Medium for about 5 minutes, stirring half way through.
While the chocolate is melting, mix 85g self-raising flour, 85g plain flour, ¼ bicarbonate of soda, 200g light muscovado sugar, 200g golden caster sugar and 25g cocoa powder in a big bowl, mixing with your hands to get rid of any lumps. Beat 3 medium eggs in a bowl and stir in 75ml (5 tbsp) buttermilk.
Now pour the melted chocolate mixture and the egg mixture into the flour mixture, stirring just until everything is well blended and you have a smooth, quite runny consistency. Pour this into the tin and bake for 1 hour 25- 1 hour 30 minutes – if you push a skewer in the centre it should come out clean and the top should feel firm (don’t worry if it cracks a bit). Leave to cool in the tin (don’t worry if it dips slightly), then turn out onto a wire rack to cool completely.
When the cake is cold, cut it horizontally into three. Make the ganache: chop 200g good quality dark chocolate into small pieces and tip into a bowl. Pour a 284ml carton of double cream into a pan, add 2 tbsp golden caster sugar, and heat until it is about to boil. Take off the heat and pour it over the chocolate. Stir until the chocolate has melted and the mixture is smooth.
Sandwich the layers together with just a little of the ganache. Pour the rest over the cake letting it fall down the sides and smoothing to cover with a palette knife. Decorate with grated chocolate or a pile of chocolate curls. The cake keeps moist and gooey for 3-4 days.