Prepare the Chicken:
Cut the chicken into bite-sized pieces. In a bowl, combine flour, garlic powder, onion powder, paprika, salt, and pepper. In another bowl, whisk together the egg and buttermilk. Place cornstarch in a third bowl.

Coat the Chicken:
Dredge each piece of chicken first in the flour mixture, then dip it into the egg mixture, and finally coat it in cornstarch. Press gently to ensure an even coating.

Fry the Chicken:
Heat about 2-3 inches of vegetable oil in a large pan or deep fryer over medium-high heat. The oil should reach about 350°F (175°C). Carefully drop the chicken pieces into the hot oil in batches, making sure not to overcrowd the pan. Fry the chicken for 4-6 minutes, or until golden brown and crispy. Use a slotted spoon to remove the chicken and place it on a paper towel-lined plate to drain excess oil.

Make the Bang Bang Sauce:
In a small bowl, whisk together the mayonnaise, sweet chili sauce, sriracha, honey, and rice vinegar until smooth and well combined. Taste and adjust the spice level if needed by adding more sriracha or sweet chili sauce.

Toss the Chicken in the Sauce:
Once all the chicken pieces are fried, place them in a large bowl. Drizzle the bang bang sauce over the chicken and toss gently to coat the pieces evenly.

Serve:
Transfer the coated chicken to a serving platter and garnish with chopped green onions, sesame seeds, and cilantro if desired. Serve immediately as a main dish or appetizer.