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1
Combine the Saikyo miso, mirin, and sake in a saucepan. Stir over low heat until the miso is fully dissolved into a smooth, uniform marinade. Remove from heat and allow to cool completely before the fish contacts it. The marinade is ready when it is homogenous — no lumps of miso remain.
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2
Lay the cod fillets in a non-reactive container. Pour the cooled marinade over them, turning each fillet so all surfaces are coated. Cover and refrigerate for a minimum of 2 days; 5 to 7 days yields deeper miso penetration. The marinade will take on a faint amber tinge from the fish fat as the days pass — this is correct.
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3
Remove the fillets from the marinade. Using clean cloth or paper, wipe every trace of surface miso from each fillet. This step is where the dish lives or dies: residual miso on the surface will burn under high heat before the fish fat has time to caramelize, yielding a bitter char rather than the lacquered mahogany crust the preparation demands. The wiped surface should be moist but visibly clean of paste.
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4
Preheat a broiler to its highest setting. Place the fillets skin side down on a rack positioned close to the element. Broil 3 to 4 minutes on the skin side — the fatty skin renders, and the base caramelization begins. The skin is ready to turn when it shows a deep, even amber color and the edges of the fillet have begun to go opaque.
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5
Turn the fillets and broil the flesh side for a further 3 to 4 minutes. The dish is done when the surface reaches a deep mahogany — the Maillard reaction on miso sugars and fish fat — and the interior yields to light pressure with the give of a set, fully cooked fillet. The smell at this point is sweet, caramel-savory, with no acrid note.