To prepare and velvet the chicken, in a bowl, combine the sliced chicken thighs with the salt and mix well. Add the white pepper, sugar, soy sauce, and dark soy sauce and mix again. In a small bowl, mix the cornstarch with the water to form a slurry, then add it to the chicken and mix until evenly coated. Add the cooking oil to seal the marinade and prevent sticking during stir-frying. Mix again and let marinate while preparing the other ingredients.
In a bowl, mix cornstarch and water to form a slurry. Then add soy sauce, dark soy sauce, oyster sauce, hoisin sauce, and chicken stock. Mix well.
To cook the chicken, in a wok over high heat, add 1 tablespoon of cooking oil. Once hot, add the chicken in a single layer and let it sear briefly before stirring. Stir-fry for about 3 minutes until just cooked through. Add the garlic and ginger and stir-fry for about 10 seconds until fragrant. Remove everything from the wok.
To stir-fry the vegetables, in the same wok over high heat, add the remaining 1 tablespoon of cooking oil. Add the broccoli, carrot, and snap peas and stir-fry for about 2 minutes until crisp-tender.
To bring everything together and serve, return the chicken to the wok. Stir the sauce again, pour it into the wok, and cook until the sauce thickens and lightly coats everything. Add the bean sprouts and toss briefly. Turn off the heat and serve immediately.