In a small bowl, whisk together ingredients for the voodoo sauce. Set aside.
In a separate bowl, whisk together buttermilk, salt, pepper, onion powder, and garlic powder. Add peeled and deveined shrimp to bowl and toss to thoroughly coat. Let sit for 15 minutes.
Remove shrimp from marinade a few at a time, letting excess marinade drip off. Dip both sides in cornstarch until fully coated. Tap off the excess and arrange breaded shrimp baking sheet.
Meanwhile, preheat broiler and toast bread lightly, about 1 1/2 minutes.
Heat 1 1/2 inches of oil in a dutch oven or electric fryer to 375 degrees. Fry shrimp in batches, cooking for 2 1/2 minutes until golden brown and crispy. Use a strainer to transfer shrimp to a paper towel lined baking sheet.
Transfer shrimp to a medium sized bowl and pour over voodoo sauce. With a spoon, carefully fold the shrimp into the sauce until all are coated.
Build the sandwich. First, add lettuce, then tomato, then pile on the shrimp. Cut the sandwich in half, on an angle, and serve.