To prepare crabs, break off tail flaps on the underside and discard. Carefully pull off back shells and reserve. Remove ‘dead man’s fingers’ (soft gills) and discard, then rinse and discard stomach sac.
Cut each crab in half lengthwise, then crack large claws to ensure they cook through. Pat crabs dry using paper towel.
Heat oil in a wok over high heat.
Add crabs in batches and cook, turning occasionally, for 2 minutes or until just before cooked through. Using tongs, remove crabs and drain on paper towel.
Remove half the oil from wok and discard. Add peppercorns, garlic, ginger, half the spring onions and half the chillies to the wok and cook, stirring, for 1 minute or until fragrant.
Add soy sauce, oyster sauce, sugar, butter and chicken stock, and simmer for 1 minute.
Return crab to wok and simmer, turning occasionally, for a further 4 minutes or until sauce has reduced.
Top with remaining spring onions, chillies and coriander leaves, and serve immediately.