Add chicken, soy sauce, sesame oil, garlic powder, and pepper to a bowl
Let sit 5 minutes while you heat a skillet with oil or cooking spray
Add chicken in a single layer and cook over medium about 3 minutes until dark and caramelized
Flip and cook another 2 minutes until cooked through
Remove chicken from pan
Add leftover marinade to the skillet with brown sugar and stir until dissolved
Turn heat to high and bring to a bubble
Mix cornstarch and water until smooth, then stir into the sauce
Reduce heat to low and let thicken about 30 seconds
Add chicken back in and toss to coat
Serve with rice and veggies
