Peel and devein the shrimp if needed. Rinse briefly under cold running water to remove any remaining shell fragments or residue from deveining. Drain and pat dry.
Place the shrimp in a medium bowl and pour the buttermilk over top, making sure they're fully submerged. Let sit for 10-15 minutes.
In a small bowl, whisk together the Japanese mayo, sweet chili sauce, and sriracha until combined. Set aside.
In a shallow bowl, whisk together the cornstarch, salt, garlic powder, and black pepper.
Remove up to 3 shrimp at a time from the buttermilk, letting the excess drip off. Coat the shrimp evenly in the cornstarch mixture, gently pressing it on. Shake off any excess. Transfer the coated shrimp to a parchment-lined baking sheet in a single layer.
Heat 1⁄2 inch of vegetable oil in a large, heavy-bottomed pan over medium heat until it reaches 350°F. Fry the shrimp in a single layer, working in small batches, for about 1 1⁄2 minutes per side until golden, crispy, and opaque throughout. Transfer each batch to a wire rack in a single layer to drain and keep the shrimp crispy. Remove any browned bits from the oil and let the oil return to 350°F between batches.
Transfer the fried shrimp to a large bowl and pour the sauce over top. Gently toss with a rubber spatula until evenly coated. Garnish with green onions and serve immediately.