Toast and grind the sesame seeds. Put the sesame seeds in a small pan and turn the heat to low. Keep stirring the seeds until they become golden – about 6 to 7 minutes. Turn the heat off and put the seeds in a sesame seed grinder or mortar and pestle. Grind the seeds into a bowl and add the soy sauce, sugar, dashi powder, and water. Stir and set aside.
Blanch the spinach. Fill a large pot with water and 1 tablespoon salt, and bring to a boil.Add the spinach and boil for 1 minute and drain.
Shock the spinach. Fill a medium to large bowl with ice and cold water. Transfer the cooked spinach to the bowl with ice and leave for a few minutes, until the spinach is cold.
Drain well. Drain the spinach and using your hands, take a chunk of spinach and squeeze as much excess water as you can. Place the spinach on a cutting board and repeat with the remaining spinach.
Cut. Cut the spinach into 2-inch long pieces and transfer it to a mixing bowl.
Mix and serve. Add the sesame sauce on top and using chopsticks, toss until the spinach is evenly coated with the sauce. Serve.