These wonderful mushrooms are said to have been cultivated for more than a thousand years in southern China, and there are many grades of them. Black or brown in colour, they add a particular flavour and aroma to Chinese dishes. The best are very large with a light colour and a highly cracked surface; they are usually the most expensive. As you can imagine, they are very popular in Hong Kong, and dried-food shops there carry every grade heaped in mounds, with the more expensive grades elaborately boxed. Here, they can be bought in boxes or plastic or cellophane bags from Chinese shops and are expensive. Keep dried mushrooms stored in an airtight jar in a cool, dry place. Fresh mushrooms of a similar variety are Japanese shiitake but they are not an adequate substitute as Chinese cooks never use them fresh, preferring the distinct, robust, smoky flavours and succulent texture of the dried variety. In Hong Kong, they are used as seasonings, finely chopped and combined with meats, fish and shellfish.