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Heat the oven to 160C/fan 140C/gas 3. Butter and line a 20cm round cake tin (7.5cm deep).
Put 200g chopped dark chocolate in a medium pan with 200g butter.
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 for about 5 minutes, stirring halfway through.
Mix 85g self-raising flour, 85g plain flour, ¼ tsp bicarbonate of soda, 200g light muscovado sugar, 200g golden caster sugar and 25g cocoa powder, and squash out any lumps.
Beat 3 medium eggs with 75ml buttermilk.
Pour the melted chocolate mixture and the egg mixture into the flour mixture and stir everything to a smooth, quite runny consistency.
Pour this into the tin and bake for 1hr 25 – 1hr 30 mins. If you push a skewer into 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. Cut the cold cake horizontally into three.
To make the ganache, put 200g chopped dark chocolate in a bowl. Pour 300ml 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. Cool until it is a little thicker but still pourable.
Sandwich the layers together with just a little of the ganache. Pour the rest over the cake letting it fall down the sides and smooth over any gaps with a palette knife.
Decorate with 50g grated chocolate or 100g chocolate curls. The cake keeps moist and gooey for 3-4 days.