Season chicken with garlic powder, salt, and pepper. In a large skillet, add olive oil and brown chicken on both sides until cooked through, about 3-4 minutes per side.
As chicken is cooking, prepare sauce. In a medium bowl, whisk together soy sauce, water, brown sugar, rice vinegar, garlic powder, ginger, and cornstarch.
Add sauce to skillet with cooked chicken and cook over low heat until thickened, about 2-3 minutes, tossing to coat.
Serve chicken over white rice with steam broccoli, if desired.