Cassia is the bold, reddish-brown bark of the Chinese cinnamon tree, sold and used as ordinary "cinnamon" across most of the world. Thicker, harder and more pungent than delicate Ceylon bark, its hot-sweet, coumarin-rich punch is the flavour most people picture as cinnamon — the backbone of cinnamon rolls, chai, mulled cider, Chinese five-spice and countless braises.