The tonka bean is the dried, black, wrinkled seed of a tall South American rainforest tree, intensely fragrant with notes of vanilla, almond, cherry, caramel and freshly mown hay. A single bean is grated in pinches over custards, creams and chocolate — a little goes a very long way — and it carries a famous asterisk: its perfume comes largely from coumarin, a compound the United States bans from food.