In a large bowl, mix chicken, cornstarch, baking powder, and 2 tablespoons vegetable oil; marinate for 10 minutes.
Place noodles in a large bowl and pour over plenty of hot water to cover. Soak for 15 minutes, then quickly rinse in cold water.
Prepare the sauce by mixing tamarind purée, palm sugar, fish sauce, soy sauce, and dark soy sauce in a saucepan over medium heat and boil until it thickens, about 2 minutes.
Heat 5 tablespoons vegetable oil in a large nonstick pan over high heat. Add egg and scramble for 1 minute. Add chicken and cook about 2 minutes, or until chicken and egg is about 80% cooked. Add noodles and scramble until soft.
Add sauce, vinegar, and chicken stock and mix together well. Add bean sprouts, scallions, and tofu, stirring occassionally for 2 more minutes until everything is incorporated.
Garnish with crushed peanuts and serve immediately with lime wedges on the side, along with crushed dried chile if you like it spicy.