Rinse school prawns. If using live prawns, fill a bucket with water and fill with about 20-30 ice cubes. Quickly transfer prawns to ice slurry and leave 5 minutes to put prawns to sleep. Drain prawns. Pat dry with kitchen towel and dust in corn flour seasoned with 1 tbsp sea salt.
Heat oil in a wok or large, deep, heavy-based frying pan. To test heat of oil, one prawn should bubble immediately and turn golden in 1 minute. In 3 to 4 batches, cook prawns until crisp and golden. Transfer to kitchen paper and repeat with remaining prawns, adding a little more oil as needed.
In a mortar and pestle, grind together lime zest and chilli flakes until slightly ground but still textured, add remaining salt and grind a little more. Sprinkle over hot prawns and serve with lime wedges for squeezing.
To make alioli, place garlic and sea salt in a mortar and pestle, found to a fine paste (2-3 minutes). Add egg yolk and stir with pestle to combine. Adding a few drops of olive oil at time, then in a thin steady stream, add olive oil and stir with pestle until thick and well combined. Season to taste with freshly ground black pepper. Use as a dipping sauce for crispy prawns.