If using katsuo bushi flakes from package, moisten with a few drops of sake or water. Finely chop katsuo bushi, then cook in a dry heavy skillet over moderate heat, stirring frequently, until fragrant and mostly dry.
Add sake, sugar, and soy sauce and cook over moderate heat, stirring frequently, until mixture is dry and glazed. Stir in sesame seeds and spread on a plate to cool.
Rinse rice in a bowl in several changes of cold water until water is almost clear; drain well in a large sieve.
Combine rice and 2 cups water in a 3-quart heavy saucepan and let stand 10 minutes. Cover with a tight-fitting lid and bring to a boil over high heat. Cook at a rapid boil (lid will be rattling and foam may drip down outside of pan) 5 minutes, or until water is absorbed.
Remove from heat and let stand, covered, 10 minutes.
Serve rice with bonito-flake topping.
·Bonito-flake topping keeps in an airtight container at room temperature 1 week.
