Open the squid out and pat dry with kitchen paper. Lay them on a chopping board, shiny-side down, and, using a sharp knife, lightly score a fine diamond pattern on the flesh, being careful not to cut all the way through. Cut the squid into 5 x 2 cm (2 x 1 inch) pieces and place in a non-metallic dish. Pour over the lemon juice, cover and chill for 15 minutes.
Combine the cornflour, salt, pepper, chilli powder and sugar in a bowl. Dip the squid pieces into the beaten egg whites and then into the cornflour mixture, shaking off any excess.
Pour the oil into a deep-fat fryer or large saucepan to a depth of at least 7 cm (3 inches) and heat to 180–190°C (350–375°F), or until a cube of bread browns in 30 seconds. Deep-fry the squid in 3 batches for 1 minute, or until it turns pale golden and curls up. Remove each batch with a slotted spoon and drain on kitchen paper.
Mix all the ingredients for the dipping sauce in a bowl. Serve the squid in small paper cones, if liked, garnished with sliced red chilli and spring onions and accompanied by the dipping sauce.