Finely chop the bonito flakes and add to an 8-inch nonstick skillet set over medium heat. Cook, stirring constantly until the flakes are fragrant, dry and toasted, approximately 4 minutes. Add the mirin, soy sauce, and sugar and continue to cook stirring constantly until the mixture is dry and appears glazed, approximately 2 to 5 minutes.
Remove from the heat and stir in the sesame seeds. Transfer the mixture to a plate and spread out to cool. Serve over rice, noodles or fish.