For this recipe it's important that you use Japanese short-grain white rice, also sometimes called sushi rice. Brown rice and long-grain rice will not work. Cook the rice according to the package directions along with the salt. While the rice is cooking bring the soy sauce and mirin to a boil over low heat until it's reduced by about half. Take it off the heat and whisk in the butter.
When the rice is done and cooled down just a bit (it should still be hot), cut off a square piece of plastic wrap and pile 1/4 of the cooked rice in the center. Wrap the plastic around it and use your hands to press it together into 1-inch thick triangle. Make sure they are densely packed together so they won't fall apart on the grill. Repeat with the remaining rice and plastic wrap.
Preheat the grill to medium.
Use a pastry brush to baste 1 side of the rice balls with the butter mixture and put them on the grill, buttered side down.
Let them grill until browned and crisp on 1 side, about 5 minutes) then brush the tops of the rice balls with the butter mixture. Flip them over using tongs and grill until the second side is crisp.
Transfer the rice balls to a serving platter.
Brush again with the butter mixture and garnish with chives.
