In a medium pot, combine ½ cup soy sauce, ¼ cup mirin (or sake), and ¼ cup water. Add a piece of dried kombu and 1 tablespoon sugar, stirring until dissolved. Bring the mixture to a boil over medium heat, then reduce the heat to low and let it simmer for 2 minutes. Remove from heat and allow the marinade to cool completely.

Carefully separate the egg yolks from the whites, placing the yolks gently in a single-layer glass container, ensuring they do not touch each other.

Once the marinade has cooled, strain it through a fine mesh sieve to remove the kombu. Gently pour the strained marinade over the yolks in the glass container, ensuring it covers them completely while avoiding direct contact.

Cover the container tightly with plastic wrap or a lid and refrigerate it. Allow the yolks to cure for at least 12 hours, flipping them gently every 12 hours. The maximum cure time is 48 hours.

When ready to serve, prepare your short-grain rice. Drizzle the finished rice with sesame oil, then garnish with chopped scallions and crushed nori, if desired. Finally, place the soy cured egg yolk on top.