Cut crabs into halves. Remove and crack the pincers. Sprinkle corn flour and pepper mixture over the crab pieces.
Heat enough oil in a wok and deepfry crab for 2 minutes until fragrant.
Dish out and drain oil. Heat butter in a wok, and when it starts to melt, add in curry leaves, lemongrass and ground ingredients.
Fry for a few minutes until fragrant. Put in combined sauce ingredients and crabs. Stir-fry until crabs are evenly coated with the sauce. Close the wok with a cover and cook for 23 minutes until crabs are fully cooked and the gravy is thick.