dashi powder
Instant stock made from dried bonito flakes, usually with kombu, salt and flavour enhancers, sold as granules or powder; dissolved in hot water it becomes Japan's basic stock.
Dashi powder is the dried, instant form of dashi, the stock on which miso soup, simmered dishes, sauces and many batters in Japanese cooking are built. It consists mainly of powdered katsuobushi, that is smoked and dried bonito, often with kombu seaweed, salt, sugar and glutamate added; there are also fish-free versions based on kombu and shiitake. It tastes smoky, faintly fishy and strongly savoury, noticeably more pronounced and saltier than homemade dashi, so it is used sparingly and other salt is added only after tasting. It is sold in Asian grocery shops as granules in jars, pouches or single-portion sticks, and by mail order; kept dry and unopened, it lasts a long time.
Substitute:
The best replacement is homemade dashi from kombu and bonito flakes, which tastes more delicate and less salty, so you will need to season more. Kombu alone, or kombu with dried shiitake, makes a fish-free stock but lacks the smoky note. Supermarket chicken or vegetable stock is not a real substitute: the dish stays edible but no longer tastes Japanese.