Cut the chicken into 1-inch chunks and season with 1 teaspoon garlic powder, 1 teaspoon salt, and 1/2 teaspoon black pepper; set aside.
In a small pot, combine 1/2 cup soy sauce, 1/4 cup water, 1/4 cup brown sugar, 1/4 cup honey, 1 tablespoon rice vinegar, 2 teaspoons minced garlic, 1 teaspoon minced fresh ginger, and 1 teaspoon sesame oil.
In a small bowl, stir 1 tablespoon cornstarch with 2 teaspoons cold water until smooth, then pour the slurry into the pot and stir to combine.
Bring the sauce to a rolling boil, cook 1 minute, then reduce heat and simmer 3–4 minutes until slightly thickened; remove from heat (it will thicken more as it cools).
Heat 1 tablespoon sesame oil in a large skillet over medium-high heat until shimmering.
Add the seasoned chicken to the hot skillet in a single layer; let sit without stirring for 3 minutes, then stir and cook 2–3 more minutes until the chicken reaches 165°F (74°C) internally.
Remove the chicken from the skillet and toss with the prepared teriyaki sauce until evenly coated.
Garnish with finely chopped green onions and serve immediately.
