Trim stems frombroccoli. Cut chicken breast crosswise on an angle into 1 cm-thick slices. In a small bowl, toss chicken with cornstarch, soy sauce, and lemon extract. In a wok or frying pan, heat oil 10 seconds over medium heat. Add broccoli and stir-fry to coat with oil. Add 2 tb water, reduce heat to low, cover and cook until broccoli is just tender, about 2 minutes. With a slotted spoon remove to a plate. Return wok to high heat. Add ginger and scallion and stir-fry until aromatic, about 10 seconds. Add chicken and stir-fry until white and opaque throughout, about 5 minutes. Add sherry, pepper, sugar, and broccoli. Stir-fry about 30 seconds to heat through. Remove from heat, drizzle with sesame oil, and serve.