Heat the oven to 425°F. Set a wire rack over a foil-lined rimmed sheet pan and coat the rack lightly with oil.
Blot the wings very dry, then toss with baking powder and salt until no visible powdery patches remain.
Arrange with space between pieces. Bake 25 minutes, turn, and bake 18 to 25 minutes more until deeply browned.
Measure several thick pieces close to but not against bone. Continue cooking until every checked wing reaches at least 165°F.
Simmer soy sauce, water, brown sugar, mirin, garlic, ginger and sesame oil for 3 minutes. Whisk in the slurry and simmer briefly until glossy and lightly thickened.
Rest the wings for 5 minutes, toss with enough warm glaze to coat without pooling, then add scallions and sesame seeds.