1.Cook soba noodles in a large pot of boiling salted water until tender; drain and rinse under cold water. Divide among serving bowls.
2.Bring water or stock to a boil with wakame and shiitake mushrooms; simmer 4 minutes, or until mushrooms are tender.
3.Ladle some of the boiling water over miso in a heatproof bowl; stir to loosen miso.
4.Add dashi powder and prawns to soup. Reheat gently, without boiling, about 5-10 minutes, so the prawns gently poach in the soup.
5.Once prawns are cooked through, add miso mixture, tofu and green onions. Stir well to combine.