Carefully separate the egg yolks from the whites. Place the yolks in a small container.
Pour in enough soy sauce to fully cover the yolks, then add mirin. Optionally, stir in a little sugar for sweetness.
Refrigerate and cure:
4–6 hours for soft, jammy yolks
Longer (up to 24 hours) for slightly firmer, sliceable yolks
Fully firm (up to 48 hours) - this option is the best for grating or longer storage