Put the milk in a saucepan and slowly bring to the boil. Cool to 45°C (113°F). Stir in the yoghurt or dried culture. Add the skim milk powder, if desired, to make a thicker, creamier yoghurt.
Cover, then wrap in a thick cloth and set aside in a warm place at a constant temperature, between 40°C and 46°C (104°F and 115°F), for 5–6 hours, or until s