How to Make It
In a small saucepan, stir the dashi powder into 1 cup of water until dissolved. Add the sake, soy sauce, mirin and sugar and bring to boil. Simmer over moderately low heat until syrupy and reduced to 1 cup, about 1 hour. Transfer all but 1/3 cup of the sauce to a jar and reserve for later use.
In a large skillet, heat the peanut oil over moderate heat until shimmering. Add the duck breasts and cook until almost medium-rare, about 4 minutes per side. Add the reserved 1/3 cup of sauce to the pan and cook the duck breasts for 4 minutes longer, swirling the pan and turning the duck once or twice until glazed. Remove from the heat and let rest for 5 minutes.
Transfer the duck to a platter and drizzle with the glaze from the pan. Sprinkle the duck with the scallions and sesame seeds and serve.
The sauce can be refrigerated for up to 1 week. Rewarm before using.