In a bowl, whisk together the cornstarch, Chinese five-spice, 1 tsp salt, and 1 tsp ground black pepper until evenly combined.
Toss the cubed chicken with the cornstarch mixture until each piece is evenly coated.
Heat 1 tablespoon of sesame oil in a cast iron skillet over medium-high heat until shimmering.
Add the coated chicken to the hot skillet in a single layer and cook, stirring or turning frequently, until golden brown on all sides and cooked through, about 8–10 minutes depending on piece size.
Transfer the cooked chicken to a plate and, if needed, blot with paper towel to remove excess oil.
Add the remaining 1 tablespoon sesame oil to the skillet and stir in the diced green onions (reserve a small amount if desired for garnish), diced red chili, and diced green chilies. Cook for about 2 minutes until fragrant and slightly tender.
Return the chicken to the skillet, toss with the aromatics, and cook for 1–2 minutes more to combine and reheat the chicken. Adjust seasoning with extra salt or pepper if desired, then serve immediately.