Double steaming is a Chinese cooking technique to prepare delicate food such as bird nests, shark fins etc. The food is covered with water and put in a covered ceramic jar. The jar is then steamed for several hours. This technique ensures there is no loss of liquid or moisture (its essences) from the food being cooked, hence it is often used with expensive ingredients like Chinese herbal medicines.
Cantonese calls double steaming dan. Note that the Cantonese usage of this Chinese character deviates from its original meaning which is boil or simmer in Mandarin. It is unclear if this cooking technique is common outside of Cantonese cuisine, thus, there may not be a non-Cantonese term for it.
Cantonese cuisine is famous for its slow cooked soup. One famous dish of this kind is called the Winter melon urn (冬瓜盅). It is prepared by emptying the inside of a winter melon to make an urn. The outside of the winter melon is often carved with artistic patterns. The inside is then filled with soup ingredients such as Chinese cured ham, and several Chinese herbs. Winter melon is believed to be nournishing and it is seldom cooked with ingredients that are believed to be too yin or too yang. The whole urn completed with its original melon lid is double steamed for at least four hours. The flavor of the soup is soaked into the "meat" of the melon. The whole melon and its content is brought to the dinner table. The soup is served by scoping out the liquid and the inside wall of the melon. In this case, the edible melon takes the place of the double steaming jar. This application is possible because winter melon has a waxy, thus waterproof, rind.
There is another dessert dish called double steamed frog ovaries in a coconut (椰青燉雪蛤膏) designed for women. The Chinese medicinal ingredients, rock sugar are placed inside a young coconut soaking in the original coconut juice. The filled coconut is then double steamed for several hours. The whole coconut is served whole at the table after dinner. The contents and the inside wall of the coconut are scooped out to be consumed.