First, prepare the marinade. In a saucepan, add 200 ml water.
Add 150 ml soy sauce and 120 ml mirin. Simmer gently over low heat.
Prepare a bowl of room‑temperature water. Once the marinade comes to a boil, place the entire pot into the water to cool.
Next, cook the eggs. Bring a pot of water to a boil with 8 eggs. If your eggs are cold from the fridge, soak them in water first to bring them to room temperature.
Once the water is boiling, gently add the eggs. Cook for 6½ minutes for a firmer jammy yolk, or 6 minutes for a softer, runnier yolk.
Prepare an ice bath (cold water). When the eggs are done, turn off the heat and immediately transfer them into the ice bath to stop cooking and make peeling easier.
Once slightly cooled, peel all the eggs and rinse briefly under drinking water.
Place the eggs into a container and pour in the cooled marinade. Make sure the eggs are fully submerged. Refrigerate overnight.
The next day, enjoy your perfectly marinated jammy eggs.