To make the prawn mixture, toss the prawns with 1 tablespoon cornflour, then rinse in cool water and pat dry with paper towel.
Roughly chop the prawn meat, divide in half and very finely chop half of it.
Place the prawn meat into a bowl, sprinkle the salt over the top and mix until the prawn mixture becomes elastic.
Add the remaining ingredients and mix well.
Spread 2 teaspoons of the prawn mixture over each bread triangle, making sure to spread it all the way to the edges.
Dip the bread, prawn side down, into the sesame seeds to coat.
Pour the oil into a wok or a small saucepan (it should be about one-third full) and heat to 180C (350F) or until a cube of bread dropped in the oil browns in 15 seconds.
Reduce the heat to low.
Working in batches, add the toasts, prawn side down, and cook for 1 minute.
Turn them over and cook for a further 15–30 seconds until the toast is golden.
Remove with a slotted spoon and drain on a wire rack or paper towel.
Serve hot garnished with spring onion and with mayonnaise or ketchup on the side.
