Dou fu pi (豆腐皮, tofu skin, also called yuba in its Japanese form) is the thin film of protein and fat that forms on the surface of hot soy milk when it is simmered — the same phenomenon as the skin on heated dairy milk, but made from soy protein. Dou fu pi is lifted from the soy milk with a flat stick, draped over the stick to drain, and dried to varying degrees: fresh tofu skin (fresh fu pi, 鲜腐皮) is pliable, soft, and delicate; semi-dried tofu skin (tofu sheets, 腐竹, fu zhu — literally bamboo tofu, for the cylindrical shape formed when the stick is held vertically) is slightly stiff and must be rehydrated before use; fully dried sheets are brittle and must be soaked for 30 minutes before use. Tofu skin is used extensively across Chinese cooking — wrapped around fillings (素鸡, su ji, Buddhist vegetarian chicken), in braises, in soups, and in Jiangsu's thousand-layer tofu preparations.
Making fresh tofu skin: Simmer homemade soy milk (thick — from 200g soybeans per 1L water) in a wide, shallow pan over very low heat. Do not stir — the skin needs undisturbed soy milk surface to form. After 3-5 minutes, a wrinkled skin will form. Slide a flat stick under the skin from one edge to the other. Lift slowly and drape over the stick above the pot. The skin is fresh fu pi. Cooking with rehydrated dried tofu skin (fu zhu): Soak dried tofu skin sticks in cold water for 30-60 minutes until completely pliable. Cut into sections. Use in braised dishes — they absorb braising sauces readily and develop a pleasant, slightly chewy texture.
Fuchsia Dunlop, Land of Fish and Rice (2016); Eileen Yin-Fei Lo, Mastering the Art of Chinese Cooking (2009)