Bring a pot of water to a boil. Once boiling, gently lower the eggs. Turn down the heat to medium-high and boil for 7 minutes for a soft-boiled yolk consistency 4.
Transfer the boiled eggs into a bowl filled with ice cold water to cool down completely.
Meanwhile in a sauce pot, combine together the soy sauce, mirin, sake, and sugar. Bring this mixture to a boil, then let simmer for 1 minute, stirring well to dissolve the sugar. Remove from heat and let cool completely.
Peel your cooled, boiled eggs and transfer to a container of your choice. Pour in the cooled soy marinade, then place a paper towel or plastic wrap on top to fully submerge the eggs.
Let the eggs marinate for at least 8 hours to overnight 5.
