Cut crabs into halves. Clean well and crack the claws. Sprinkle a little cornflour over the crabs and deep-fry in hot oil until the crabs turn red. Dish out and drain from oil.
Combine evaporated milk and egg yolks and mix well. Put butter in a wok and melt over medium heat. Pour the egg yolk mixture gradually into the melted butter at a height, allowing it to drip in a steady stream. Stir constantly to fry the egg yolk mixture until it forms fine shreds. Add the curry leaves and bird's eye chillies. Stir-fry until fragrant.
Return the crabs to the egg mixture. Season with sugar and salt. Stir well to mix. Pour into a metal sieve to drain off excess melted butter or the oil. Transfer to a serving dish and serve immediately.