Put water and kombu in a pot and let it sit for at least 30 minutes until the kombu softens.
Heat the pot over very low heat for about 10 minutes (for 2 servings). When small bubbles appear on the surface of the liquid, turn off the heat and remove the kombu. Kombu dashi is ready.
Sprinkle sake over the salmon fillets and let them sit for about 5 minutes.
Remove the scales from the salmon and cut the fillets into bite-sized pieces. Cut the potato, carrot, naganegi, and cabbage into bite-sized pieces.
Bring the kombu dashi in the pot to a boil, then add the salmon, carrot, and potato. Reduce the heat to low and simmer for about 10 minutes until they become tender, occasionally skimming off any scum.
Add the naganegi, cabbage, and sake to the pot and bring it to a brief boil. Turn off the heat, then dissolve the miso in the pot using a ladle and chopsticks.
Serve in each bowl and top with butter, if desired.