Season chicken with salt and pepper. Marinate in buttermilk for 10–15 minutes while heating the oil.
Heat 1–2 inches of vegetable oil in a deep skillet or pot over medium-high heat to 350°F (175°C).
Dredge chicken pieces in cornstarch, coating well. Shake off excess.
Fry chicken in batches for 3–4 minutes until golden and crispy. Drain on paper towels.
In a bowl, whisk together mayonnaise, chili sauce, sriracha, honey, and vinegar (if using) until smooth.
Toss hot chicken with the Bang Bang sauce until evenly coated.
Garnish with green onions and sesame seeds. Serve over rice, in wraps, or on a salad.