Preheat deep fryer with peanut oil to 190°C.
Whisk together buttermilk, egg, and mustard in a casserole dish. In another casserole dish, whisk together flour, cornmeal, paprika, 2 teaspoons salt, cayenne, and garlic powder.
Dredge prawns in the dry mixture then into the wet then back into the dry. Add prawns in batches to the deep fryer and fry until golden, about 2 minutes per batch. Serve with spicy mayo sauce.
Add the shallot, parsley, cayenne pepper, mayonnaise, mustard, hot sauce, and lemon juice to a bowl. Stir to combine. Chill until ready to serve.