Stir together buttermilk and sriracha. Place shrimp in the buttermilk marinade and allow shrimp to marinate for 10-15 minutes.
Prepare the Bang Bang Sauce. In a separate bowl, whisk together mayonnaise, sweet chili sauce, rice vinegar, sriracha sauce, and sugar. Taste and adjust the spice level with more sriracha if needed. Set the sauce aside.
Whisk together cornstarch, kosher salt, black pepper, garlic powder, and onion powder.
Remove shrimp from the buttermilk marinade and dredge the shrimp in the seasoned cornstarch mixture, making sure they're evenly coated.
Heat the vegetable oil in a deep pan or skillet over medium-high heat.
Fry the coated shrimp in batches until they are golden brown and crispy, about 2 minutes per side. Place the fried shrimp on a wire rack to allow them to cool; using a wire rack keeps the shrimp crispy.
While the shrimp is cooking, dice your tomatoes and slice your green onions.
In a large bowl, drizzle the crispy shrimp with the prepared Bang Bang Sauce. Toss the shrimp to coat them evenly with the sauce.
Garnish the Bang Bang Shrimp with chopped green onions and tomatoes.
Serve your homemade Bang Bang Shrimp as an appetizer or over a bed of white rice or fresh lettuce.