Gather the ingredients.

Coat chicken pieces with cornstarch in a bowl; set aside. Beat eggs, salt, and pepper in a mixing bowl until combined. Stir in flour and baking powder until no large lumps remain. Mix in chicken pieces until evenly coated.

Heat vegetable oil in a large skillet or wok over high heat. Cook chicken in hot oil until golden brown and no longer pink on the inside, about 12 minutes. Set chicken aside; keep warm.

Make sauce: Reduce heat to medium-high; add green onion, ginger, and sesame oil to the skillet. Cook and stir until onion is limp and ginger begins to brown, about 1 minute. Pour in water, sugar, and vinegar; bring to a boil

Dissolve cornstarch in soy sauce in a small bowl and stir into vinegar mixture along with oyster sauce and ketchup. Cook until sauce has thickened and is no longer cloudy. Stir in chicken; simmer until heated through.