Place chicken pieces in a bowl with buttermilk. Let marinate for at least 10 minutes while you prep the coating and sauce.
In a small bowl, whisk together mayonnaise, sweet chili sauce, sriracha, honey (if using), and vinegar or lime juice. Taste and adjust spice level as needed. Set aside.
In another bowl, mix flour, cornstarch, garlic powder, paprika, salt, and pepper. Remove chicken from the buttermilk and dredge each piece in the flour mixture until well-coated.
Heat oil in a deep skillet or saucepan over medium-high heat. Fry chicken in batches for 4–5 minutes, or until golden and cooked through. Drain on a paper towel-lined plate.
Place cooked chicken in a large bowl and drizzle with bang bang sauce. Gently toss to coat all the pieces evenly.
Serve warm, topped with green onions or sesame seeds if desired. Great with rice, noodles, or in lettuce wraps.