Whisk together buttermilk and egg. Add shrimp, toss until evenly coated. Chill for 30 minutes.
In a gallon size plastic storage bag toss together panko, flour, cornstarch, salt, lemon pepper, garlic and onion powder. Set aside.
Remove shrimp from chilled buttermilk, draining in a colander to allow excess to drip off. Add to breading in plastic bag, seal and toss until evenly coated. Open bag and chill for 30 minutes, then repeat sealing and tossing to coat with breading about halfway through and again just before frying..
To prepare: Heat vegetable oil in a deep pot or dutch oven to 360-365°F.
Working in batches, cook shrimp until golden and crispy turning as needed for about 2-3 minutes total. Transfer to a paper towel-lined sheet pan or platter.
Meanwhile, to make the sauce, whisk together mayonnaise, sweet chili sauce, sriracha and honey in a small bowl. Set aside.
Serve immediately, drizzled with sweet chili sauce and additional on the side. (May toss shrimp lightly with sauce, if preferred)