Pat the chicken dry and cut it into roughly 1-inch pieces.
Add ½ teaspoon salt and ¼ teaspoon black pepper, tossing until evenly seasoned.
Toss the pieces in 1 cup cornstarch until fully coated. Shake away excess and rest for 5 minutes.
Whisk orange juice, orange zest, soy sauce, brown sugar, rice vinegar, garlic, and ginger together. Keep the cornstarch slurry separate.
Add about 1½ inches of oil to a deep skillet or wok and heat to 350°F (175°C).
Cook in 2–3 batches for about 4–6 minutes per batch, turning occasionally. Chicken should be golden and reach 165°F (74°C) internally.
Transfer the cooked pieces to a wire rack and drain for 3–5 minutes.
Bring the orange mixture to a gentle simmer for 2–3 minutes. Whisk the slurry again, stir it into the sauce, and cook another 1–2 minutes until glossy.
Add the crispy chicken to the sauce and toss gently for 30–60 seconds.
Remove from the heat, garnish with sliced green onions and sesame seeds, and serve immediately.