Put the milk, salt, butter and lemon zest in a pan. Bring to the boil and slide in the pasta. Stir while you bring the milk back to the boil. Turn the heat down to the minimum so that the milk will gently simmer. Cover the pan and cook until the milk has all been absorbed and the pasta is cooked, about 30 minutes. Cooked in this way the pasta will be quite soft, not al d