In a large bowl, combine the flour, cornstarch, baking powder, garlic powder, onion powder, salt, black pepper, and cayenne pepper. Mix well.
In another bowl, pour the buttermilk over the chicken pieces, ensuring they are well-coated. Let the chicken marinate for at least 30 minutes in the refrigerator.
Heat about 2 inches of vegetable oil in a deep pot or fryer to 350°F.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge each piece in the flour mixture, pressing lightly to adhere.
Fry the chicken in batches for about 10-12 minutes, or until golden brown and cooked through (internal temperature should reach 165°F). Remove and drain on paper towels.
In a saucepan over medium heat, combine gochujang, honey, soy sauce, rice vinegar, minced garlic, and sesame oil. Stir until well combined and heated through.
Toss the fried chicken in the sauce until evenly coated.
Serve hot, garnished with sesame seeds and chopped green onions.