Peel and finely grate the ginger and garlic, then deseed and finely chop the chilli. Very finely slice the chives and prawns, then place it all into a bowl with the crabmeat, oyster sauce and sesame oil. Mix well to combine.
Lay the wonton wrappers on a clean work surface and cover with a damp tea towel to stop them drying out. Lightly dust a tray with cornflour and place a small bowl of water alongside it. Spoon 1 teaspoon of the filling onto the middle of a wrapper, brush the edges with a little water, then bring them up over the filling, pinching them to seal. Place on the flour-dusted tray, then repeat with the remaining ingredients – you should end up with 30 in total.
Pour 5cm of boiling water into a wok or saucepan over a medium-high heat and bring to the boil. Cut out a circle of greaseproof paper so it fits snugly into a bamboo steamer, grease one side with oil, then place oil-side up into the steamer. Add the wontons in a single layer (you’ll need to do this in batches), place the basket on top of the pan, cover and steam for 7 to 8 minutes, or until cooked through. Serve with chilli sauce.