Begin by draining any excess liquid from the soft tofu with care. Handle it gently to maintain its delicate texture. Once drained, use a sharp knife to cut the tofu into small cubes, approximately 1.5 cm on each side. Keeping the tofu pieces small ensures a more pleasant texture in the soup.

In a small saucepan, combine the cold water, kombu, and bonito flakes. Set the pot over medium-high heat and bring it to a boil, uncovered. As soon as the liquid reaches a boil, promptly remove the kombu to prevent it from imparting an overly bitter taste to the broth.

Reduce the heat to the lowest setting, cover the pot, and let the mixture simmer gently for five minutes. Meanwhile, prepare a straining setup by placing a clean paper towel inside a fine mesh sieve and resting it over a large bowl or measuring cup.

After simmering, pour the broth through the paper towel-lined sieve to remove all the bonito flakes. This creates a clear, flavorful dashi base. Return the strained broth to the saucepan.

Place the pot over medium heat and whisk in the miso paste, ensuring it dissolves fully into the broth. Whisking helps break down the paste quickly and evenly. Once the miso is fully incorporated, stir in the dried wakame seaweed and allow it to rehydrate and bloom.

Finally, gently add the cubed tofu to the soup. Let everything heat through briefly, being careful not to boil. Serve warm, garnished with finely chopped green onions if desired.