Put the egg white into a bowl with a little salt and pepper and whisk with a fork until frothy. Sift in the cornflour and whisk to mix, then add the chicken and stir until coated.
Heat the oil in a wok until very hot, but not smoking. Add about one-quarter of the chicken strips and stir to separate. Stir-fry for 30–60 seconds, until the chicken turns white on all sides. Lift out with a slotted spoon and drain on kitchen paper. Repeat with the remaining chicken. Very carefully pour off all but about 1 tablespoon of the hot oil from the wok.
Return the wok to a low heat and add the green pepper, chilli, garlic and about half of the spring onions. Stir-fry for a few minutes, until the pepper begins to soften, then add the black bean sauce and stir to mix. Pour in the stock, increase the heat to high and bring to the boil, stirring constantly.
Add the chicken to the sauce and cook over a moderate to high heat, stirring frequently, for 5 minutes. Taste for seasoning. Serve hot with egg noodles, garnished with the remaining spring onions and the black beans.