In a pot, bring sake and mirin to a boil over medium heat.
Turn the heat to low then add the miso paste. Whisk until the miso paste has dissolved. Add the sugar and whisk to combine well. Remove from heat and let cool completely.
Place the cod in a bowl and pour 1 tbsp sake. Let soak for 5-10 minutes then pat the cod with a paper towel to soak up any excess sake.
Pour half the miso marinade on the bottom of an air tight container.
Place the cod in the container in one layer, making sure they do not stack. Pour the remaining half of the miso marinade on top of the cod. Using a spoon or spatula, spread the marinade so it fully coats the fish.
Cover with plastic wrap and and cover with the air tight lid. Place in the fridge to marinate for 2-3 days.
When you're ready to cook the fish, wipe off the excess miso marinade from the cod with your hands or paper towel.
Lay the cod on a lined sheet tray and broil for 8-10 minutes or bake for 12-18 minutes at 400°F / 200ºC until fully cooked.
Serve with a fresh bowl of rice and your favorite side dishes.
