To cook egg, bring water to a boil and gently drop cold eggs into boiling water. Reduce heat to keep water and a gentle boil. Cook egg for about 9 minutes or to your desired doneness.
Remove eggs from boiling water and drop into a bowl of ice water to cool.
Mix soy sauce, mirin and water in a measuring cup. Peel eggs and submerge in the soy sauce mixture.
Refrigerate until ready to use (half a day or overnight).