Cuff a vacuum seal bag a couple of times and add the seasoned cod fillets. Add the butter pats and distribute them evenly in a single layer on top of the fish. Uncuff and vacuum seal the bag using a vacuum sealer. If you don't have a vacuum sealer, use another air removal method.
Add the vacuum sealed bag to the preheated water bath and secure with sous vide magnets to keep from floating, or place a ceramic plate or bowl on top of the bag. Cook for 30 minutes. If cooking from frozen, cook for 45 minutes.
After cooking, remove the bag from the water bath and carefully remove the fish from the bag to a rimmed serving plate. It will be so tender that it will likely fall apart a bit. That's okay! Pour the contents of the bag over the fish for a butter sauce.