In a medium pot, combine water and dashi powder. Bring the stock to a boil and remove from heat.
Whisk in the miso until it combines with the broth (no lumps). Add the tofu and wakame and return pot to the stove to heat the soup gently – do not boil again.
Stir gently to mix, ladle into bowls and top with green onions. Serve immediately, with rice on the side.