Shrimp Tempura Tacos

In the bowl of a food processor add mayonnaise, sour cream, scallions, garlic, cayenne, 1/2 teaspoon salt, 1/4 teaspoon black pepper, zest of half a lime, and 1 tablespoon lime juice and pulse until smooth. Transfer to a small bowl and refrigerate until ready to serve. Cut remaining limes into wedges for garnish. Pour oil into deep fryer or about 4" up the sides of a deep pot. Heat oil to 350 degrees F. Meanwhile, prepare tempura batter. In a large mixing bowl add flour, club soda, egg, and 1 teaspoon salt; whisk together until smooth. Rinse and pat dry shrimp. When oil has reached 350 degrees F, coat shrimp in batter. Using tongs, remove from batter and carefully place in hot oil. Cook until golden brown, about 1 minute per batch, and transfer to a paper towel–lined plate. Season shrimp with salt immediately. Fry shrimp in batches and ensure oil returns to 350 degrees F each time. Warm tortillas in a dry skillet. Assemble tacos with shredded cabbage, shrimp, and scallion aioli. Garnish with scallions and serve with lime wedges and extra sauce on the side.