In a pot, combine soy sauce, mirin, and sake
Bring sauce mixture to a boil
Lower heat and simmer for a 1-2 minutes. This will remove the alcohol content from mirin and sake.
Remove from heat and place sauce in a heat safe container
Add kombu to the sauce and let the sauce cool down to room temperature or place in fridge for use later
Once the sauce is cool, separate egg yolks from egg whites and place only the yolks in the sauce
Allow the egg yolks to marinate in the fridge for 24 hours or longer. The longer it is marinated, the firmer and jammier the yolks become
Place egg yolks over hot rice and garnish with toppings of choice
Serve and enjoy!