In a medium pot, set over medium low heat, melt the butter. Add the kombu, if using, and add the carrots. Sprinkle the carrots with about 1/4 teaspoon of salt. Cover the pot and cook until very tender, about 30 minutes. Using a fork, test the softness of the carrots. They should halve very easily with the side of a fork. Discard the used kombu.
Just a heads up, depending on the size of your blender, you may need to puree the carrots with the carrot juice in batches. Add half of the carrot juice (you can eyeball this measurement) to the blender. Pour in the cooked carrots and pulse until very smooth, adding the remaining carrot juice. Pour in 3/4 cup to 1 1/4 cup of water, depending on how you like the soup. It shouldn’t be so thick that it’s like baby food. It should be slightly drizzling.
Pour the carrot soup back in the pot and warm over medium heat. Salt to taste. (I added about 1/4 teaspoon of salt.) Turn the heat down to low to keep warm while you mix together the furikake.
In a small bowl, toss together the nori, white sesame seeds, black sesame seeds and bonito flakes. Divide the soup amongst bowls and garnish with a handful of furikake.