1. Heat the mirin in a 1-quart saucepan over moderate heat until lukewarm. Turn off the heat, ignite the mirin with a match, and shake the pan gently back and forth until the flame dies out.
2. Add soy sauce, niban dashi, katsuobushi, a pinch of salt, and sprinkle lightly with MSG. Bring to a boil over high heat, then strain the sauce through a fine sieve set over a small bowl.
3. Cool to room temperature and taste for seasoning (adding a little salt if necessary).