Sesame seeds with a black coat, used toasted. Ground with sugar and lard they fill black sesame tangyuan, and ground and cooked with water and rock sugar they make hot black sesame soup.
In traditional Chinese medicine 黑芝麻 is classified as 补肾 (kidney-nourishing) and is popularly believed to prevent grey hair - hence the ubiquity of 黑芝麻糊 (black sesame paste) and 黑芝麻丸 as health snacks.