In a small bowl, mix all the ingredients together. I like to use a fork to mash everything and incorporate it all together.
Position a rack in the center of the oven. Preheat the broiler. If your broiler has settings, set it to low. My oven doesn't have settings, so I put the rack closer to the bottom, because my broiler runs REALLY hot.
Season fish generously, on both sides, with salt and pepper.
In a cast iron skillet, or oven proof skillet, combine the miso butter, brown sugar, sake and 1/2 tablespoon of water. Cook over medium heat, until all the ingredients are incorporated.
Gently place the fish in the skillet and cook until the bottom 1/2 inch of the fish is opaque, about 3-4 minutes. Spoon some of the sauce over the fish as it cooks, coating each fillet with the miso butter.
Place skillet under the broiler and broil until the tops of the fillets are caramelized. They might start to char slightly which is even better, because this adds extra flavor. This whole process should take about 3-7 minutes,depending on the heat of your broiler. If the fish starts getting charred too quickly, turn the broiler to a lower setting, or move the rack to a lower position in your oven.
Remove pan from oven, using a spatula, and gently remove each fillet and place on a large platter or individual plates. Spoon the sauce over the fillets and garnish with fresh chives and/or parsley.
Serve with rice (white, sticky, basmati and jasmine are all great options!)