Chaga is the hard, blackened conk of Inonotus obliquus, a parasitic fungus that grows on living birch trees across the cold northern forests of the world. Charred and cracked like burnt wood on the outside but rusty-orange within, it is not eaten whole but broken up and simmered or steeped into an earthy, faintly vanilla-woody tonic tea — and has become, with reishi and cordyceps, one of the pillars of the modern functional-mushroom market.