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1
Combine dashi, mirin, and soy sauce in the ratio 3 : 1 : 1 to form the tare. The balance is mirin-forward — perceptibly sweeter than a general tsuyu. Taste: the soy should register as background salinity, not dominant. The tare is correct when it reads sweet-savoury without sharpness.
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2
Work in individual small pans — a small yukihira copper pan is ideal. For each portion, bring one quarter of the tare to a simmer over medium heat. Add one quarter of the chicken thigh pieces in a single layer. Simmer until the chicken is just cooked through and has absorbed colour from the tare — the flesh turns from pink to opaque white throughout, and the tare reduces slightly and glazes the surface.
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3
This is where the dish lives or dies. Raise the tare to 85 °C — a steady, active simmer with small bubbles breaking the surface. Pour in two thirds of the beaten egg for this portion in a slow, circular stream over the chicken. Do not stir. Allow the exterior to set into soft, just-cohesive curds while the centre remains entirely liquid. When the edges have caught and the egg mass moves as one piece, reduce heat so the pan temperature drops toward 70 °C. Pour the remaining one third of the egg into the liquid centre in a thin stream. Remove the pan from heat immediately; the centre must still tremble visibly when the pan is lifted.
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4
Mound hot rice into a round donburi bowl. Slide the topping — chicken and barely-set egg — directly over the rice in one motion. Invert the bowl over the pan for 30 seconds; carryover heat from the rice and the enclosed steam sets the top surface precisely without overcooking the interior. Lift the bowl: the surface should be just opaque, with a trembling softness beneath.
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5
Dust the surface with a small pinch of kona-sansho. Serve immediately — the topping deteriorates rapidly and the dish is made to order only. The sansho delivers a faint numbing warmth detectable at the back of the palate.