You can use the Koji method on individual steaks. The reason you can't do true dry aging on individual steaks, trimming the inedible outer layer leaves little or no steak left. You need a larger cut so you still have usable steaks after trimming. With the Koji 2-3 day method you have no inedible outer layer to trim, just a rinse. Koji is better suited to smaller cuts because of the limited time the enzymes have to work their way into the meat.