Step 1: Pat chicken thighs thoroughly dry with paper towels and cut into 1-inch pieces. Season with salt and black pepper.
Step 2: In a medium bowl, combine cornstarch and baking powder. Add the chicken pieces and toss until evenly coated.
Step 3: Heat 2 tablespoons of oil in a large non-stick skillet or wok over medium-high heat. Add chicken in a single layer (cook in batches if necessary) and cook for 5-7 minutes per side, until golden brown and cooked through. Remove chicken and set aside.
Step 4: In a small bowl, whisk together soy sauce, honey, rice vinegar, minced garlic, grated ginger, sesame oil, and red pepper flakes (if using) for the sauce. Set aside.
Step 5: In the same skillet (add 1 tbsp more oil if needed), add mixed vegetables. Stir-fry for 3-5 minutes until tender-crisp. Push vegetables to one side of the pan.
Step 6: Add a bit more oil to the empty side of the pan. Crack in the eggs and scramble until just set. Break into smaller pieces and mix with the vegetables.
Step 7: Add the cold, day-old rice to the skillet. Break up any clumps. Pour in 1 tablespoon of soy sauce and stir-fry vigorously for 3-5 minutes, until well combined and heated through.
Step 8: Return the cooked chicken to the skillet with the fried rice. Pour the prepared sauce over everything. Toss gently but thoroughly until the chicken is coated and the rice is infused with the sauce. Cook for 1-2 minutes, allowing the sauce to thicken slightly.
Step 9: Garnish with sliced green onions and sesame seeds, if desired. Serve the crispy saucy chicken with vegetable fried rice immediately.