In a bowl, whisk together flour, cornstarch, onion powder, garlic powder, salt, and pepper.
In a separate bowl, whisk together buttermilk and egg until combined. Place the panko in a third bowl.
Dredge the shrimp in the flour mixture, dip in the buttermilk mixture, then coat with panko. For extra crispy coating, repeat the dredging and dipping process.
Heat oil in a skillet over medium-high heat until a bit of flour sizzles when added.
Fry shrimp in batches, turning halfway through, until golden and crispy. Transfer to a paper towel-lined plate to drain.
In a bowl, whisk together mayonnaise, sweet chili sauce, and sriracha to make the sauce.
Toss warm fried shrimp in the sauce until coated. Garnish with scallions and sesame seeds. Serve immediately.
Notes
Double-dipping in the flour and buttermilk makes for a thicker, crunchier coating. Serve with extra sauce on the side for dipping.