If the mushrooms are large, cut into bite sized pieces. Place wok over a medium/high heat, add oil, garlic and ginger and fry until fragrant (about 30 seconds). Add the chicken and stir-fry for 2 minutes. Add mushrooms and stir fry for a further minute, add oyster sauce, dark soy sauce, salt, white pepper, sugar and chicken stock and mix well. Bring sauce to the boil, and allow to reduce by a third, this will take about 2 minutes. Pour in the corn flour slurry to thicken the sauce, stirring the entire time, remove from heat and add sesame oil. Transfer to serving dish and enjoy.