In a large bowl, whisk together cornstarch and salt. Add beef; toss to coat. Shake off any excess.
In a large skillet, heat 3 Tbsp. oil over medium-high heat. Add half of beef and fry until browned on all sides, about 5 minutes. Using a slotted spoon, transfer beef to a colander. Wipe any browned bits from skillet, and repeat using remaining 3 Tbsp. vegetable oil and second batch of beef. Wipe out skillet.
Add sesame oil, garlic and ginger (if using) to skillet and cook for 1 minute over medium heat. Stir in chiles, orange juice, marmalade, soy sauce and vinegar and cook over medium-high heat until slightly syrupy, about 4 minutes. Reduce heat to medium and stir in beef to coat with orange sauce. Heat through, 2 minutes.
Serve beef and sauce over rice.