Rinse the pak choi leaves under cold running water, then dry thoroughly with kitchen paper. Roll each pak choi leaf up and slice through thinly with a small, sharp knife, or chop using a food processor. Make sure that the leaves are finely shredded.
Heat the groundnut oil in a large wok or frying pan until hot. Carefully add the shredded pak choi to the wok, and fry for about 30 seconds until it shrivels and becomes crispy. You may need to do this in batches.
Remove the crispy ‘seaweed’ from the wok with a slotted spoon, and drain on kitchen paper. Transfer to a large bowl, toss with the salt and sugar, and serve immediately.