Put all the ingredients for the marinade in a bowl and mix well, until the miso paste has dissolved.
Add salmon fillets to the bowl, and coat well on both sides. Turn the fillets so the skin side is up and refrigerate for 30 to 60 minutes, maximum. If you leave the salmon for longer than that, the texture might get a little mushy and the flavor, too salty.
Cover a cooking tray with parchment paper and place the salmon fillets, skin side down (wipe off some of the marinade from the fillets – mainly the lumps – before placing them down to prevent the miso sauce from burning).
Turn on the broiler and move the grill to the second highest rack, about 6 inches away or more from the flame. The reason you don’t want to place the tray any closer is because miso and mirin burn easily.
Place the cooking tray on the grill and broil for 8-10 minutes, until salmon is cooked through. Keep an eye on the salmon to make sure the marinade isn’t burning.If you noticed the marinade getting darker too quickly, move the salmon to a lower rack.