In a large bowl, combine buttermilk, salt, and pepper. Add chicken pieces and toss to coat. Cover and refrigerate for at least 30 minutes, or up to 4 hours for extra tenderness.
In a small bowl, whisk together mayonnaise, sweet chili sauce, sriracha, honey, and rice vinegar until smooth. Taste and adjust spice level. Set aside.
In a shallow dish, mix flour, cornstarch, garlic powder, smoked paprika, and salt. Remove chicken from buttermilk, letting excess drip off, then dredge each piece thoroughly in the flour mixture. Shake off excess and place on a wire rack.
Heat oil in a deep pot or fryer to 350°F (175°C). Working in batches, fry the chicken pieces for 5-7 minutes until golden brown and cooked through (internal temp 165°F). Drain on a paper towel-lined plate.
In a large bowl, toss the hot crispy chicken with the prepared Bang Bang sauce until evenly coated. Transfer to a serving plate, garnish with green onions and sesame seeds, and serve immediately.