Shrimp and prawns differ in several ways, including anatomically, but when it comes to cooking, most people use them interchangeably, unless a specific type is called for (such as carabineros and tiger prawns).
When I buy them, I look for freshness and liveliness, rather than counting the number of claws they have, or checking to see how the shell segments overlap. Although I say "shrimp" in in this recipe, you can also use prawns.
The correct way to eat salt and pepper shrimp is without peeling away the shells – where all the flavour is. If the shrimp are cooked right, you can eat almost the entire thing: pull off the head, suck out the juices and nibble away the crispest bits (sometimes the whole head is edible), then eat the body, shells and all.
For this recipe, don’t buy shrimp (or prawns) that are too large, because the shells will be thicker and therefore less tender. I like small ones - the body (without the head) shouldn’t be longer than 6cm (2⅔in).
Rinse the shrimp under cool running water, then drain them. Leave the heads and shells on, but remove the veins: bend the shrimp forward into a curl, and at the base of the back, lift up the segment of the shell closest to the tail, to expose the flesh. Make a small slit in the flesh, then use the tip of a toothpick to hook the vein and slowly and carefully pull it out. Don’t tug it too fast, or it will break. If you like, trim the shrimp of the long antennae and legs. (I prefer to keep these on, because when fried, they become crisp.)
Cut the banana chillies into rings about 3mm ( ⅛in) thick, and finely mince the bird’s-eye chillies; with both types of chillies, shake out and discard as many seeds as possible as you slice them. Roughly chop the garlic and cut the spring onions about 5mm (¼in) thick.
Pour oil to the depth of about 5cm (2in) in a wok and heat to 175°C (345°F). While the oil is heating, put the cornstarch in a bowl and stir in the chilled soda water. Dip the shrimp into the cornstarch mixture then place them in the hot oil and fry until they are just cooked, less than a minute. Drain them on paper towels. Fry the shrimp in batches, and stir the cornstarch mixture frequently, so the cornstarch doesn't sink to the bottom of the bowl.
After frying all the shrimp, pour the oil from the wok into a heatproof bowl. Rinse the wok to remove any residual cornstarch, then place it over a high flame. When the wok is dry and hot, add about 15ml (1tbsp) of the oil used to fry the shrimp. Add the garlic, both types of chillies and the spring onions and stir-fry for about 30 seconds. (Try to avoid breathing in the fumes, because they will make you cough and sneeze.) Scoop the ingredients from the wok and place them in a bowl.
Heat the wok again over a high flame and add about 25ml (5 tsp) of the oil used to fry the shrimp. Put all the shrimp back into the wok and sprinkle in the salt and pepper. Stir-fry to coat the shrimp with the seasonings, then stir in the garlic, chillies and spring onions. Transfer the ingredients to a serving platter.