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1
Bring a large pot of water to a full, rolling boil. Prepare an ice bath — a bowl of water with a generous quantity of ice — and set it beside the stove. Lower the refrigerator-cold eggs gently into the boiling water using a slotted spoon. Maintain the boil and cook for exactly 7 minutes.
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2
At 7 minutes, transfer the eggs immediately to the ice bath. This is where the dish lives or dies: any delay allows carryover heat to continue setting the yolk past the custardy 63–65°C window, collapsing the gradient that defines aji-tama. Leave the eggs in the ice bath for a minimum of 5 minutes. The shell will feel cool to the touch throughout.
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3
Combine the dashi, mirin, soy sauce, and sake in a small saucepan. Warm gently over low heat, stirring to integrate. Do not boil. Remove from the heat and allow to cool completely to room temperature. The tare must be cool before the peeled eggs are introduced; warm liquid will continue to cook the whites.
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4
Peel the cooled eggs carefully under cold running water. The whites should be fully set and smooth. Place the peeled eggs in a zip-lock bag or a snug container and pour the cooled tare over them. The eggs should be fully submerged. Seal and refrigerate for 24 hours. The benchmark is a seasoned exterior penetrating approximately 3 mm, with a pure egg-custard interior remaining unmarinated — the gradient from soy-mirin umami to clean yolk is the sensory target.
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5
Remove the eggs from the tare after 24 hours — do not exceed 48 hours, as soy penetration beyond the optimal 3 mm produces an overly salty result throughout. Halve each egg lengthwise. The yolk should be orange, liquid-custard at the centre, and fully cohesive at the outer margin — not opaque, not runny. The white should be firm and carry a mahogany tint at the surface.