In a large sauce pot combine water and konbu. Place on medium high heat and remove just before the boil. Strain and immediately add ice water to cool. Add bonito flakes and place back on heat. Bring new mixture just to a boiling point and strain immediately. Season liquid with 2 tablespoons each of soy sauce and mirin.
Preheat oven 325 – 350 degrees Fahrenheit.
In a large bowl, using a hand held blender, mix together the eggs and the sea urchin. Combine with the dashi mixture and allow to rest in fridge. Discard any impurities that will settle on the top.
Place sake in a separate bowl and add chopped prawns. Allow to marinade for at least 10 minutes.
In small 2 ½ ounce ramekins add a few pieces of chopped prawns and a few shavings of water chestnuts. Pour custard over until ramekin is almost full. Top with a little chiffonade of shiso.
Cook in a bain marie or water bath for 25 – 35 minutes.
When custards are finished cooking remove from oven and allow to cool.
Mix an equal amount of soy sauce and dashi mixture. Slightly warm custards either in the oven or for 10 seconds on medium high in the microwave.
Spoon a teaspoon of the soy-dashi mixture on top and serve immediately.