Add white miso, mirin, sake, sugar, and soy sauce to a bowl.
Whisk until smooth and glossy.
Pat the cod dry using paper towels.
Spread the marinade over all sides of the fish using a spoon or basting brush.
Place the fillets in a container, cover, and refrigerate for 12 hours.
The longer it marinates, the richer the flavor becomes.
Preheat your oven to 400°F.
Line a baking sheet with foil or parchment to prevent sticking.
Brush off excess marinade from each fillet using a spoon (this keeps it from burning).
Place the cod on the sheet and bake for 12 minutes or until the fish flakes easily.
Turn on the broiler and broil for 2 minutes to caramelize the glaze.
Remove from the oven and rest for 2 minutes before serving.
