Clean crabs, remove and crack the pincers. Trim the legs and cut each into 4 pieces. Rinse and drain the crabs in a colander.
Toss crabs in tapioca flour and deep-fry in hot oil until just cooked. Remove and set aside.
Heat sesame oil in a clean wok and fry garlic, ginger and bird´s eye chillies until fragrant.
Add combined sauce ingredients. Stir well then cover the wok and allow to simmer for 1-2 minutes. Remove the lid and add the crabs. Toss to coat the crabs with the sauce. Add the beaten egg and drizzle in thickening. Dish out and serve immediately.