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.
Fill a large pot with water and 1 tablespoon salt, and bring to a boil.
Meanwhile, fill a medium to large bowl with ice and cold water. Set aside.
Add the spinach and boil for 1 minute and drain.
Transfer the spinach to the bowl with ice and leave for a few minutes, until the spinach is cold.
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 the spinach into 2-inch long pieces and transfer it to a mixing bowl.
Add the sesame sauce on top and using chopsticks, toss until the spinach is evenly coated with the sauce. Serve.