In a medium sized bowl, salt & pepper chicken, then toss together the chicken and cornstarch.
In a large skillet (or wok) heat 2 tablespoons of sesame oil over medium heat until glistening, about 2 minutes. Working in small batches, add in the chicken and allow to crisp, about 8 minutes. Turn often until all sides are golden brown and chicken is cooked through. Repeat steps until all chicken is cooked, adding more oil only if necessary.

Lay chicken on a towel to absorb oil. Once chicken is cooked add remaining 2 tablespoons of sesame oil to the pan and bring to high heat. The oil should be glistening and almost smoking. Add in green beans stirring often until blistered (little dark spots) and just barely tender, about 5 minutes. Remove from pan.

Reduce heat to medium-low, allow skillet to cool down slightly. Stir in garlic, soy sauce, chili sauce & crushed red pepper flakes. Stir until all ingredients are combined and garlic is fragrant, about 2 minutes. Add chicken and green beans back to the pan stirring until coated in sauce. Taste and season with salt if desired. Sprinkle with sesame seeds if desired.