Soak the shrimp in buttermilk for 15 minutes.
Mix cornstarch with salt and pepper in a separate bowl.
Coat shrimp in cornstarch mixture, shaking off excess.
Dip shrimp in buttermilk again, then coat with panko breadcrumbs.
Heat oil in a frying pan over medium heat.
Fry shrimp in batches until golden brown, about 2-3 minutes per side.
Drain fried shrimp on paper towels.
In a bowl, combine mayonnaise, sweet chili sauce, sriracha, and lime juice.
Toss the shrimp in the sauce until evenly coated.
Serve immediately, garnished with cilantro if desired.