Steps

1. In a bowl, combine 750ml of water and 30g of brown sugar. Stir until dissolved. [This is the key!] White sugar has a stronger sweetness, so using brown sugar or raw sugar, which are rich in minerals, is recommended.
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2. Once the sugar is dissolved, add 1 tbsp of vinegar.
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3. Add 30g of kelp to the liquid, cover with plastic wrap, and refrigerate for 2 days. [This is the key!] When rehydrating kelp, if you add soy sauce first, only the soy sauce flavor will penetrate. Rehydrate with just water and sugar. Due to osmotic pressure, sugar is the hardest to penetrate, making this order important.
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4. After 2 days, once the kelp has softened sufficiently, transfer the rehydrating liquid to a pot.
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5. Cut the softened kelp into your desired size with a knife. Japanese restaurants often cut it into squares. [This is the key!] The kelp can be slimy, so be careful not to cut your fingers.
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6. Add the cut kelp to the pot.
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7. Place the pot over high heat and bring to a boil.
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8. Once heated, add 20ml of dark soy sauce. [This is the key!] The flavor might seem weak at first, but by simmering it until the liquid reduces, the flavor will become perfectly balanced in the end.
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9. Once boiling, reduce the heat to a simmer and cook until the liquid has completely evaporated, about 30 minutes to 1 hour. [This is the key!] If the liquid evaporates in less than 30 minutes, the kelp will become tough, so always cook for at least 30 minutes. The cooking time may vary depending on your stove's heat, so watch carefully to prevent burning. You can add a small amount of water midway if needed, but it's best to keep a close eye on it over low heat.
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10. When the liquid has reduced, add 10g of bonito flakes and mix briefly.
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11. Confirm that the kelp is tender, and then increase the heat to high to quickly reduce the remaining liquid. [This is the key!] The liquid will disappear very quickly, so occasionally shake the pot to prevent it from sticking to the bottom.
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12. Once the liquid has mostly evaporated and you hear a crackling sound, continue to stir and stir-fry without stopping, moving the pot. [This is the key!] It is important to cook using the water the kelp was rehydrated in. This traps the umami from the ingredients without adding anything extra.
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13. When the liquid has completely dried and it's stir-fried, turn off the heat, plate, and serve. [This is the key!] The addition of bonito flakes creates a delicious accent with the kelp due to their excellent compatibility. If you prioritize appearance, cut everything to the same size from the beginning.
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