• about 48 square inches kombu (kelp), left over from making dashi (basic sea stock)
• 6 tablespoons rice vinegar
• ¼ cup soy sauce
• 3 tablespoons mirin (syrupy rice wine)
• generous ¼ teaspoon sanshō (fragrant Japanese pepper) powder OR 1 scant teaspoon salted sanshtō berries (see Note)
• slice the kelp into 1-inch squares.

In an enamel, glass, or stainless-steel pot, bring 2 cups of water to a rolling boil, then add 3 tablespoons of rice vinegar. Reduce the heat to maintain a steady though not vigorous boil, and cook the kelp squares for 7–8 minutes. Drain the kelp and repeat this cooking process for another 5 minutes using 2 cups of fresh water and 3 more tablespoons of rice vinegar. These acid baths help to make the kelp tender and porous, but don’t really affect the final taste. If the surface of the kelp squares seems sticky, rinse under fresh cold water and pat dry on paper towels.

Rinse your saucepan and fill it with the soy sauce and syrupy rice wine. Over medium heat, bring this mixture to a simmer, then place the parboiled kelp squares in it. Cook over fairly low heat for 4–5 minutes, stirring frequently. The liquid will become very foamy as it reduces rapidly. Season the kelp with the fragrant pepper powder, stirring to ensure even distribution. Remove the pan from the heat and let the dark, lustrous kelp squares cool to room temperature before serving or storing for future use.