Wipe the kelp with a damp cloth and bring almost to the boil with the water in a medium-sized saucepan.
Lift out the kelp. Bring the water to the boil and stir in the bonito flakes.
Remove from the heat and allow to stand for 3 minutes.
Strain through muslin and reserve.
In a clean, medium-sized saucepan, bring the stock (dashi) to a simmer.
Season with the soy sauce and stir in the cornflour paste over a low heat, stirring until the soup is thickened and smooth.
Pour the egg mixture through a sieve, circling over the soup, to make fine threads in the liquid.
Cook very gently for a few seconds longer and serve into bowls with covers, grating fresh ginger into each bowl.