Half-fill a saucepan with oil. Heat over medium-high heat. Place cornflour in a shallow dish. Lightly coat prawns in cornflour, shaking off excess. Holding by the tail, dip prawns, 1 at a time, into batter to coat. Carefully drop prawns, 4 or 5 at a time, in hot oil. Cook for 3 to 4 minutes or until golden. Transfer to a wire rack. Repeat with remaining prawns and batter.