In a bowl, combine the shrimp with the buttermilk. Cover and let marinate for at least 10 minutes (up to 30 for more flavor and tenderness).
In another bowl, whisk together the cornstarch, flour, onion powder, salt, and cayenne pepper until well combined.
In a small bowl, mix the Kewpie mayo, Thai sweet chili sauce, and Sriracha until smooth. Set aside.
Remove each shrimp from the buttermilk, letting the excess drip off. Dredge in the dry coating mix, pressing gently to ensure an even layer.
Heat vegetable oil in a deep pan to 350°F (175°C). Fry shrimp in batches, about 2 minutes per side, until golden and crispy.
Transfer cooked shrimp to a wire rack set over a baking sheet to drain excess oil and stay crispy.
While still hot, lightly toss the shrimp in the bang bang sauce
Garnish with sliced green onions.