I always make my sauce first so it's ready to go the moment my shrimp come out of the fryer. Just stir the mayonnaise, sweet chili sauce, and sriracha together in a small bowl. Set it aside on the counter while you work on the shrimp.
In a separate bowl, whisk together the buttermilk, salt, pepper, onion powder, and garlic powder. Pour this over your peeled shrimp and stir until every piece is coated. Let the shrimp sit in this marinade while your oil comes up to temperature.
Pour about 1 1/2 inches of oil into your Dutch oven or deep pot and heat it to 375 degrees Fahrenheit. I like clipping a thermometer to the side of the pot so I can keep an eye on it without guessing.
Take the shrimp out of the marinade one at a time, letting the extra liquid drip off. Dip each piece into the cornstarch, coating both sides, then tap off anything extra. Lay the coated shrimp out on a platter so they're ready to fry.
Fry the shrimp in three batches so the oil temperature doesn't drop too much. Each batch takes about 2 1/2 to 3 minutes until the shrimp turn golden brown and cook through. Use a strainer to lift them out onto a paper towel-lined plate to drain for a few minutes.
Move the warm shrimp into a mixing bowl and drizzle half of your bang bang sauce over the top. Toss gently so every piece gets coated, then let the shrimp rest for about 10 minutes so the sauce really soaks in.
While the shrimp rest, toast your corn tortillas on a griddle or skillet for about 30 seconds per side. If you have a gas stove, holding them briefly over the open flame gives a nice light char.
Layer your toppings onto each tortilla, then add two to three shrimp per taco. Finish with a drizzle of the remaining sauce, a sprinkle of cilantro, and a squeeze of fresh lime juice before serving.