Peel the prawns leaving the tail on, cut a slit at the back and remove the dirty cord. Slice further down but without cutting the prawn in 2. Flatten the flesh. Next, prepare the batter by whisking together the coconut milk, coriander lemon zest, coconut, custard powder and corn flour. Hold the prawns from the tail and dip in the batter. Shallow fry the prawns on both sides until golden brown and crispy. Pat dry on kitchen paper and sprinkle lightly with salt. Serve the prawns warm with the garlic mayonnaise on the side.