In a casserole or large pot of boiling salted water, add the crabs and cook them for about 1 minute, or until no longer moving. Drain and set aside until cool enough to handle.
To clean the crabs pull off top shells; pull and twist off each apron; remove gills. With sharp knife cut crabs into quarters, rinse and drain.
In mortor and pestle or small food processor pound or process chiles, garlic, shallots and ginger with a little oil if necessary.
Combine all sauce ingredients.
Heat a wok over moderately high heat. Add 2 tablespoons oil and heat until hot. Add chili and garlic paste and stir-fry 1 minute. Add crabs and stir-fry 2 to 3 minutes or until shells turn red. Add sauce ingredients, bring to boil and simmer, covered, 5 to 7 minutes. Transfer crabs to serving dish.
Bring sauce in wok to a boil. Stir cornstarch mixture and add to wok. Simmer until thickened. Gradually add eggs in a stream, whisking, and cook for 1 minute. Pour sauce over crabs and garnish with scallion.