Can I use 125ml of buttermilk for the new Chocolate Sheet Cake recipe, rather than the milk/vinegar option in the recipe? Many thanks.
Nigella's new recipe for Chocolate Sheet Cake (on the Nigella website) has a sponge base made using milk that has been soured with vinegar (or you can use lemon juice) as the acidity acts on the glutens in the flour and helps to give a more tender cake crumb. An acidic element is also needed in the cake batter as the leavening in the recipe is bicarbonate of soda (baking soda) and this needs an acidic element to react with to make the cake rise.
Often buttermilk is used for this reason in cake recipes but Nigella mentions that she uses the acidulated milk as the frosting in the recipe uses just plain milk and it saves having to buy and open two separate cartons. So it is fine to use buttermilk instead of the milk/vinegar mixture and in the same quantity (125ml/1/2 cup).