On Santai Mountain in Yunnan's Dehong Dai and Jingpo Autonomous Prefecture lives an ancient ethnic group called De'ang. They have a special sour tea that has been passed down for centuries.
The sour tea is made from naturally fermented tea leaves. De'ang tea makers would put the leaves in bamboo tubes and bury them in the soil for weeks before they are fully fermented. Then they take the tea leaves out and crush them in mortar on the ground with a big pestle until the leaves turn into a paste. They use a mold to make the tea paste into a cake, and dry it naturally in the sun.
The best sour tea cake is dark black with a satin-like glow on the surface. De'ang people would cut them into small pieces when they make tea.