Buttermilk – a common ingredient used in baking, though not always easy to get your hands on. A great substitute is a low-fat plain natural yoghurt, or you can make your own buttermilk by mixing 250ml/9 fl oz/1 cup milk (dairy or plant-based) with 1 tablespoon lemon juice or vinegar. Leave for 5–10 minutes to thicken and appear to curdle.
