Carefully separate the yolks from the whites.
Place the yolks in a suitable container so that they can be filled with marinade.
Mix mirin and soy sauce.
If there is no mirin, then a white semi-sweet wine will do.
Close the container and refrigerate for 8 hours.
For 8 hours, the yolks are covered with a strong shell, and inside there will be a thick yolk soaked in sauce.
And if you keep it for a day, the yolks will become amber in color and even thicker.
Together with rice, it is very tasty.