Boil eggs: Place eggs in a small pot and add enough water to cover them. Bring it to a boil over medium heat, then reduce the heat to low and let it simmer for 8-10 minutes. Drain the hot water and replace it with cold water to cool the eggs.
Make the sauce: In a pot, mix soy sauce, mirin, sugar, and water. Bring it to a boil until the sugar dissolves, then turn off the heat.
Remove the shells: Peel the eggshells and transfer the eggs into a freezer bag.
Marinate: Pour the sauce into the bag with the eggs and seal the bag, removing excess air. Store it in the refrigerator to marinate for 1-3 hours.
Notes
Storage: Store nitamago in an airtight container in the fridge for three days.
Adjust the boiling time to your liking. Boil for 8 minutes for a soft-boiled egg or 10 minutes for a hard-boiled egg.
After boiling the eggs, cool them in a cold water bath with ice to stop the cooking process and make peeling easier.
Rotate the eggs during marination once to ensure that each part absorbs the flavors evenly.