In a large bowl, combine the chicken pieces, soy sauce, sesame oil, garlic powder, onion powder, ground ginger, salt, and pepper. Mix well and let marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more flavor.
Preheat a Blackstone griddle or large skillet over medium-high heat. Add the vegetable oil and let it heat up.
Once the oil is hot, add the marinated chicken to the griddle. Cook for about 5-7 minutes, stirring occasionally, until the chicken is cooked through and has a nice sear.
While the chicken is cooking, add the mixed vegetables to the griddle. Cook for an additional 5-7 minutes, stirring frequently, until the vegetables are tender-crisp.
Add the butter to the griddle and toss everything together until the butter is melted and coats the chicken and vegetables.
Remove from heat and garnish with chopped green onions before serving.