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Stir all the ingredients for the mayo together, adding lime juice to taste. Set aside.
Coat the prawns in ½ tbsp Cajun spice mix. Decant the buttermilk into a bowl and give it a good stir to loosen slightly. In another bowl, mix the polenta, cornflour, mustard powder and remaining Cajun spice mix together.
Fill a deep frying pan 2 inches full with oil. Heat to 180C on a cooking thermometer, or until a piece of bread browns in 20 seconds when it is dropped in the oil. Dip each seasoned prawn briefly in the buttermilk then coat in the flour mixture. Fry the prawns for 1-2 mins on each side until golden brown and curled. Drain on kitchen paper, then serve with the mayo and lime wedges for squeezing over.