Boil water in a pot, add 1 tsp salt (not listed in ingredients), and blanch the water spinach starting with the stems for 30 seconds, then immerse the whole bunch for 1 minute. Transfer to cold water and drain. Cut into 4 cm pieces.

In a pot, combine soy sauce, sake, mirin, dashi powder, and water. Bring to a boil, then turn off the heat and add the blanched water spinach. Let it cool.