Combine the teriyaki sauce ingredients: in a liquid measuring cup or small bowl whisk together 1/4 cup soy sauce, 1/2 cup brown sugar, 1 cup water, 1/4 cup honey, 2 1/2 tablespoons cornstarch, 1/4 teaspoon ground ginger, 1/2 teaspoon garlic powder, and 1 tablespoon sesame seeds until the cornstarch is fully dissolved and the mixture is smooth. Set aside.
Pat the 2 pounds boneless, skinless chicken breasts dry and cut them into even pieces (about 1-inch cubes) so they cook uniformly.
Season the chicken pieces with 1 teaspoon garlic powder and salt and pepper to taste; toss to coat evenly.
Heat a large skillet over medium heat. Add 1 tablespoon olive oil and let it warm until shimmering.
Add the seasoned chicken to the hot skillet in a single layer (work in batches if needed to avoid overcrowding). Cook, stirring or turning occasionally, until the chicken pieces are cooked through and no pink remains.
Reduce heat to medium-low and pour the prepared teriyaki sauce over the cooked chicken. Stir to combine, bring to a gentle simmer, and cook, stirring frequently, until the sauce thickens and becomes glossy.
Remove from heat, sprinkle with extra sesame seeds for garnish, and serve.