Boil sake and mirin over high heat and reduce by 1⁄3.
Turn heat down, add miso and sugar, and cook for 2 to 3 minutes, stirring constantly to prevent burning.
Cool to room temperature.
Marinate cod fillets in the miso mixture for 4 days.
Gently wipe off excess marinade from the fillets and grill or broil until well browned.