Bring 4 cups of dashi stock to a gentle simmer in a medium pot.
If using dried wakame, rehydrate in cold water for about 5 minutes, then drain and squeeze out excess water.
Cut silken tofu into 1/2‑inch cubes and keep chilled until ready to add.
Lower the heat so the stock is hot but not boiling to avoid breaking the tofu.
Place miso paste in a small bowl or ladle, add a few tablespoons of hot dashi and whisk until smooth to dissolve the miso.
Remove the pot from direct heat and stir the dissolved miso back into the pot to preserve flavor and enzymes.
Gently add the tofu cubes and rehydrated wakame to the miso broth and warm through for 1–2 minutes.
Taste the soup and adjust seasoning with a splash of soy sauce or mirin if desired.
Add thinly sliced scallions just before serving for freshness.
Ladle into bowls and serve immediately; avoid boiling after adding miso to keep delicate flavors intact.