Steps

1. Gently wipe the surface of the 10g kombu with a lightly dampened paper towel. [Key Tip!] Do not wipe off the white powdery spots on the kombu, as they are a source of umami.
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2. Place the water 1L and wiped kombu in a pot and let it soak for at least 1 hour. [Key Tip!] If you have time, soak it the night before and store it in the refrigerator.
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3. Place the pot over low heat.
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4. When the edges of the pot start to show small bubbles, remove the kombu. [Key Tip!] The ideal temperature is when steam just begins to rise (around 60°C or 140°F). Boiling will release bitterness and off-flavors.
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5. Slightly increase the heat of the pot to medium.
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6. Turn off the heat and gently add the 15g bonito flakes. [Key Tip!] Start by using thinly shaved bonito flakes, like hana-katsuo (flower bonito).
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7. Let it sit for about 5 minutes to extract the umami and aroma from the bonito flakes.
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8. Line a sieve with paper towels and gently strain the ichiban dashi. [Key Tip!] Do not squeeze the bonito flakes; let the liquid drip naturally (squeezing can release bitterness).
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9. Return the kombu and bonito flakes used for ichiban dashi to the pot.
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10. Add 500cc of water to the pot.
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11. Place the pot over medium heat and warm it up just before boiling. You can stir it with chopsticks to help break up the ingredients.
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12. When it's just about to boil, reduce the heat to very low and extract the umami from the kombu for about 3 minutes.
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13. After 3 minutes, turn off the heat.
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14. Add the 5g bonito flakes for chasing bonito and let it sit for about 5 minutes to extract the umami.
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15. Line a sieve with paper towels and gently strain the niban dashi.
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16. Prepare the 150g drained silken tofu and finely chop the 1 leek.
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17. Pour 500cc of ichiban dashi into a pot and heat over low to medium heat.
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18. Add the silken tofu to the cold dashi and warm it up.
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19. Once it boils, turn off the heat and add the finely chopped leek. [Key Tip!] Add the leek after turning off the heat to prevent its aroma from dissipating.
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20. Using a strainer or whisk, dissolve the 2 tbsp miso (approximate amount). [Key Tip!] Straining the miso removes any remaining grains of rice koji, resulting in a smoother texture.
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21. Taste and if it's good, gently reheat it to warm it up without boiling (do not boil). [Key Tip!] Be careful not to boil the miso after adding it, as boiling will cause the flavor and umami to disappear.
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22. Cut the leftover kombu from ichiban and niban dashi into bite-sized pieces.
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23. Place the cut kombu and the leftover bonito flakes in a frying pan.
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24. Add 1 tbsp soy sauce, 2 tbsp cooking sake (rice wine), 2 tbsp mirin (sweet rice wine), and 1 tbsp sugar.
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25. Place the frying pan over low to medium heat and cook until the liquid has evaporated.
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26. Once the liquid has been thoroughly reduced, it is ready. Enjoy!
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