Stir the mayonnaise, chili sauce, and sriracha together in a small bowl to mix well. Set aside.
Place the shrimp in a mixing bowl with the buttermilk, salt, black pepper, and garlic powder and toss to coat.
Place the cornstarch in a shallow bowl and dredge the shrimp in the cornstarch until fully coated, making sure to let the buttermilk drip off each shrimp before dredging. Set aside on a wire rack.
Heat about ¾ inch of the oil in a pot on medium until it reaches 375 F.
Fry the shrimp in the oil in small batches for 3-4 minutes until golden brown. Do not overcrowd the shrimp.
Remove the shrimp from the oil with a slotted spoon and set them aside on a dish lined with paper towels.
Transfer the shrimp to a mixing bowl and pour ¾ of the sauce over them. Toss to coat the shrimp evenly.
Warm the tortillas in a dry skillet on medium for about 30 seconds per side.
Assemble the tacos by placing the cabbage on the tortillas and topping it with shrimp, tomato, scallion, and cilantro.
Drizzle the remaining sauce over the tacos and serve immediately.