Five spice powder is a cocoa colored blend of five ground spices, usually anise seed, fennel, clove, cinnamon, and ginger or pepper. It has a slightly sweet, pungent flavor and should be used sparingly.
Cut beef across prominent grain into 1/8 inch thick slices; cut each slice into 2 inch pieces. Combine soy sauce, sherry, ginger, garlic, sugar, five spice powder, and red pepper in a large bowl. Toss the beef with soy sauce mixture. Set aside.Separate bok choy leaves from stems; rinse and pat dry. Stack leaves and cut into 1 inch slices. Cut stems diagonally into 1/2 inch slices.Drain beef; reserve marinade. Combine reserved marinade and cornstarch in a small bowl; stir until smooth. Set aside.Heat the wok over medium high heat until hot. Drizzle 1 tablespoon oil into the wok and heat for 30 seconds. Add beef; stir fry 2 minutes or until beef is barely pink in center. Remove beef to large bowl.Add remaining 1 tablespoon oil. Add bok choy stems; stir fry 3 minutes. Add bok choy leaves; stir fry 2 minutes.Stir marinade mixture until smooth; add to the wok. Stir fry 1 minute or until sauce boils and thickens.Return beef and any accumulated juices to the wok; heat through.Serve over noodles. Garnish is desired.