Prepare the crab by cutting away the face (this can taste bitter), slicing just behind the eyes. Next lift the flap on the underside of the crab and cut this off. Loosen the top “shell” of the body and remove the gills. Rinse the crabs well.
Mix together all the marinade ingredients in a bowl, then add the crabs, cover with plastic wrap and marinate for 30 minutes.
Fill wok half way with peanut oil. Place a wok over medium-high heat and heat the oil to 180° or until a piece of bread turns golden brown in 15 seconds and floats to the surface.
In a shallow bowl, stir together two heaped tablespoons sweet potato starch with the beaten eggs (batter can have some lumps). Remove the crabs from the marinade. Dip the crabs in the batter and then dredge again in remaining sweet potato flour. Carefully lower crabs into the hot oil using a slotted spoon. Deep fry until golden brown, about 3 to 4 minutes. Drain on paper towels and season with salt and chili flakes while they are still hot.
Serve immediately over Mango Noodle Salad and garnish with chives and red chilies.