Clean the crab and cut into pieces. Pat dry with paper towels and put into a big bowl. Add the corn flour to the bowl and lightly coat the crab pieces with it. Heat up a wok and add cooking oil. When the oil is heated, drop the crab pieces and deep fry. As soon as they turn red, dish out, strain the excess oil and set aside.
Heat up a wok and add 1 tablespoon of cooking oil. Add the ginger and stir-fry until aromatic. Put the crab pieces into the wok and quickly stir a few times before adding the sauce. Add the chopped scallions, toss the crab in the wok a few times until well coated with the sauce, dish out and serve immediately.