Cut chicken thighs into uniform 1-inch pieces and pat completely dry with paper towels. Season lightly with salt and pepper.
In a shallow bowl, beat the eggs. In a separate large bowl, whisk together cornstarch, flour, salt, and black pepper.
Pour vegetable oil into wok or deep skillet to about 2 inches deep. Heat oil to exactly 375°F (190°C), using a thermometer to monitor temperature.
Dip each chicken piece into beaten egg, letting excess drip off. Transfer to flour mixture and toss to coat completely, shaking off excess flour.
Add 6-8 coated chicken pieces to hot oil without overcrowding. Fry for 3-4 minutes, turning occasionally, until golden brown and internal temperature reaches 165°F (74°C).
Remove fried chicken with slotted spoon and place on wire rack over paper towels. Let oil return to 375°F between batches. Repeat with remaining chicken.
In a medium saucepan, heat sesame oil over medium heat. Add minced garlic, grated ginger, and red pepper flakes. Sauté for 30 seconds until fragrant.
Add water, rice vinegar, soy sauce, granulated sugar, brown sugar, fresh orange juice, and orange zest. Bring to a simmer over medium heat, stirring occasionally.
Stir cornstarch slurry to recombine, then slowly pour into simmering sauce while stirring constantly. Cook for 1-2 minutes until sauce thickens to a glossy, syrupy consistency.
Remove sauce from heat and immediately add all fried chicken pieces. Gently toss with tongs or spoon until every piece is evenly coated.
Transfer orange chicken to a serving platter over steamed rice. Garnish with sliced green onions and sesame seeds. Serve immediately while still crispy.