'Chocolate cake absolutely has to be two things: dark and moist. A dry chocolate cake hits you in the back of the throat with its coarse crumbs, like a tickly cough. To make sure it is sticky and dense I use buttermilk. This might seem a curious ingredient to include in a chocolate cake, but it is completely necessary to guarantee a moist and tender crumb. Essentially sour milk (not the same thing as soured, spoiled milk!), its acidity reacts with the bicarbonate of soda to give this cake its rise. It also acts as a tenderiser, stopping tough gluten networks forming so quickly. If you can’t find buttermilk, mix some milk with lemon juice as a substitute. The juice will make the milk curdle, but don’t worry about this.
'Adding coffee to chocolate cakes really brings out the flavour in the cocoa, and you can barely taste it. If you detest coffee or are baking for children, you can use boiling water instead in any of the chocolate cake recipes.'
This recipe is taken from Martha Collison’s new book Twist.