Prep the chicken: Pat the chicken dry and cut into even pieces.
Season with salt and pepper. In a bowl, whisk the egg, then toss in the chicken to coat.
Coat for crispiness: Mix cornstarch and flour in a shallow dish. Dredge the egg-coated chicken in the mixture, shaking off excess.
The coating should be even and light.
Whisk the sauce: In a bowl, combine orange juice, zest, soy sauce, sugar, vinegar, garlic, ginger, and chili sauce if using. Taste and adjust sweetness or salt as needed.
Pan-fry the chicken: Heat 1/4 inch of oil in a large skillet over medium-high. Add chicken in a single layer, working in batches.
Cook 3–4 minutes per side until golden and cooked through. Set on a wire rack or paper towels.
Reduce the sauce: Pour the sauce into a clean skillet or wipe the pan and reuse it. Bring to a simmer.
Stir in the cornstarch slurry and cook 1–2 minutes until thick and glossy.
Combine and finish: Add the chicken to the sauce and toss to coat. Drizzle in sesame oil. Cook 1 minute more to let the coating absorb the sauce slightly.
Garnish and serve: Top with green onions and sesame seeds.
Serve hot over rice with a side of veggies.