General tips
Marinate in white miso for a full 24 h. The miso needs that time to penetrate past the surface and flavor the flesh through. Less than 12 h and you've got a surface coating; more than 48 and you start to cure the fish (which is its own thing).
Use white miso, not red. Red miso (aka miso) is too aggressive for fish and will overpower the cod's mild flavor. White miso (shiro miso) is fermented for less time and has a sweeter, more delicate profile that complements the fish instead of fighting it.
Sous vide at 122°F for 45 min. The bath sets the flesh while the miso flavor - already locked in by the long marinade - gets carried throughout. Anything longer and the cod starts to break down from the salt content of the miso.
Broil for 90 sec at the end, not longer. The miso surface caramelizes fast under high heat - you want golden brown, not black. Watch it like a hawk; the difference between perfect and burned is 15 sec. Pull as soon as the top blisters.
Anti-tip: don't wipe off the miso before broiling. That layer of glaze is the whole point - it caramelizes into the iconic Nobu-style finish. Just give it a light scrape with a butter knife if it's too thick, but leave a generous coat on top.
Delicate tips
Marinate fish for full 48 h for authentic Nobu-restaurant depth - the long marinade is key to the flavor
Scrape off excess marinade before bagging - thick globs burn during broiling and taste bitter
Watch the broiler like a hawk when caramelizing - miso sugar burns very quickly
Firm tips
This temperature is more forgiving and easier to handle than delicate 120°F - good for beginners
Fish holds together much better for plating and presentation
Still needs careful broiling to caramelize miso without burning