Preheat oven to 350°F (175°C). Whisk together all sauce ingredients (buffalo sauce, brown sugar, apple cider vinegar, melted butter, soy sauce, garlic powder, chili powder, mustard powder, and salt) in a bowl until well combined. Set aside.
Cut the chicken into 2-inch pieces and pat dry with paper towels. Season generously with salt and pepper.
Whisk the eggs vigorously in a bowl until air pockets form and the color is uniform. Add the chicken pieces and toss to coat.
Heat vegetable oil in a large skillet or Dutch oven to 365°F (185°C). Working in batches, remove chicken from the egg, roll in cornstarch to coat, then briefly dip back into the whisked egg.
Fry the chicken in batches for 3-4 minutes per side until golden brown and crispy. Do not crowd the pan. Transfer fried chicken to a wire rack set over a baking sheet.
Transfer the fried chicken to a large bowl and pour the sauce over the top. Toss until every piece is evenly coated.
Arrange the sauced chicken in a 9x13-inch baking dish. Bake for 10 minutes, stir to re-coat, then bake for an additional 10 minutes.
Remove from oven and garnish with sliced green onions and sesame seeds. Serve immediately over steamed rice.