Trim the ends off the cucumbers, slice them diagonally into thin 1/8-inch slices, and place them in a zip-top bag.
In a small bowl, combine the water, rice vinegar, soy sauce, sugar, salt, and dashi powder. Stir until the sugar and dashi powder are dissolved.
Pour the marinade over the cucumbers, then add the sliced red chili peppers and white sesame seeds.
Remove as much air as possible from the bag and seal it tightly.
Gently massage the bag to evenly coat the cucumbers with the marinade.
Refrigerate for at least 15 minutes before serving. If you’ve ever wondered how long it takes for cucumbers to pickle, they’ll already be flavorful after a short rest, but overnight creates an even better balance.