Ginkgo nuts
The seed kernel of the ginkgo tree, shelled and cooked: jade-green to yellowish, soft yet slightly chewy, mild with a faintly bitter undertone.
Caution: Ginkgo nuts contain a neurotoxin and should only be eaten cooked and in small amounts, with about ten a day generally regarded as the limit for adults and considerably fewer for children.
Ginkgo nuts are not nuts in the botanical sense but the seed kernels of the ginkgo tree, enclosed in a hard, pale shell and a thin brown skin. Once cooked they turn a bright green to yellow and take on a peculiar texture somewhere between chestnut and firm jelly, slightly starchy and a little sticky. The flavour is mild and buttery, threaded with a gentle bitterness that is part of their appeal. In Japanese cooking they are mainly an autumn ingredient, used in savoury egg custard, in rice, or roasted and salted as a small snack. Asian grocers usually sell them shelled and precooked, vacuum-packed or canned, and sometimes fresh in the shell in autumn; they can also be ordered online.
Substitute:
There is no real substitute, since the combination of chewy bite, colour and quiet bitterness is not found elsewhere. Edamame supply the green and chestnut pieces some of the starchy texture, but both taste sweeter and different; in most dishes it is better simply to leave the ginkgo nuts out.