Whisk together mayonnaise, sweet chili sauce, sriracha, honey, and lime juice. Taste, adjust sriracha, and refrigerate until needed.
Whisk buttermilk and egg in a medium bowl. Add shrimp, toss to coat, and marinate for 15 minutes.
Mix cornstarch, flour, garlic powder, paprika, salt, and black pepper in a wide shallow bowl.
Heat oil in a heavy-bottomed pan to 175°C (350°F). Use a thermometer.
Lift each shrimp from the marinade, let excess drip off for 2 seconds, then press firmly into the coating on both sides.
Fry 6 to 8 shrimp at a time for 2 to 3 minutes, turning once, until golden-brown and shrimp reach 63°C (145°F) internal temperature.
Drain on a wire rack. Keep finished batches warm in a 120°C (250°F) oven.
Add all fried shrimp to a large bowl, pour over three-quarters of the sauce, and toss to coat. Plate, garnish with spring onions and sesame seeds, and serve remaining sauce on the side.