Make mayo by combing the first 3 ingredients in a small bowl. Cover and refrigerate until needed.
Heat the oil in your deep fryer to 350 degrees F. Preheat your oven to 200 degrees F. Set a wire-rack over a baking sheet.
Place the flour, cornstarch, baking powder, salt, paprika and pepper in a small to medium bowl and whisk to combine. Pour the beer into a second small to medium bowl. Add the flour mixture to the beer and whisk until a smooth batter forms.
Working in batches, dip the prawns, holding on to the tail, into the batter, draining away the excess. Deep-fry until golden brown, about 3 to 4 minutes. Set the cooked prawns on the wire rack; keep warm in the oven until the rest are deep-fried. Arrange prawns on a serving plate with the mayo alongside. Garnish with lime slices and serve.
Note: Devein and butterfly the prawns after peeling. To do so, with a paring knife, cut almost all the way through the flesh down the centre of each prawn. Pull out or rinse the dark intestinal vein. Open the flesh of each prawn until it sits flat.