In a medium bowl, combine shrimp with buttermilk and soak for about 10 minutes.
In a separate bowl, mix Japanese mayonnaise, sweet chili sauce, and sriracha for the bang bang sauce.
Whisk together cornstarch, salt, garlic powder, and black pepper in a shallow bowl.
Dredge shrimp in the cornstarch mixture, ensuring both sides are coated.
Heat vegetable oil in a skillet over medium heat and fry shrimp until golden brown, about 1.5 minutes per side.
Place cooked shrimp on a wire rack or paper towel-lined plate, drizzle with bang bang sauce, toss to coat, and garnish with green onions.