Heat the sesame oil over medium heat in a large heavy-bottomed skillet. Add the ginger and garlic and cook for about 1 minute, stirring occasionally.
Add the soy sauce, rice vinegar, brown sugar, and ketchup to the skillet and stir to combine.
Create a slurry by whisking the cornstarch and chicken broth (or pineapple juice) together until fully mixed. Add the mixture to the skillet and bring to a soft boil, remove from heat.
Reserve a 1/2 cup of the marinade and set it aside. Place the chicken in a baking dish or pan and pour the remaining marinade over the chicken and toss it to coat it completely.
Follow through with cooking the chicken immediately or place it in the fridge to marinate for 30 minutes up to 6 hours.
Set the grill to medium-high (or place a grill pan on the stove set to medium-high) and oil the racks.
Cook the first side of the chicken for 2 to 3 minutes or until golden and flip to the other side and cook for 2 minutes.
Baste the already cooked side with the reserved marinade and then flip the chicken to that side to cook for another minute. Repeat this process every minute for about 10 minutes until the thickest part of the chicken registers 165ºF.
Transfer the chicken to a place and cover loosely with foil and let rest for 5 minutes before serving (optional).