Place the dried flat rice noodles in a large bowl. Pour over very hot water from the tap and leave to soak for 20–30 minutes, or until they’re pliable but still have a slight bite. Drain well and set aside.
Whisk together the tamarind paste, fish sauce, light brown sugar, rice vinegar and Sriracha (if using) in a small bowl until the sugar has dissolved. Taste and adjust to ensure a balanced sweet, sour and salty flavour.
Heat 1 tbsp vegetable oil in a wok or large frying pan over medium-high heat. Add the chicken and stir-fry for 3–4 minutes until almost cooked through. Add the prawns and cook for 2–3 minutes, until pink and opaque. Remove both from the pan and set aside.
Add the remaining 1 tbsp oil to the pan. Stir in the garlic and shallot, cooking for 30 seconds until fragrant. Push them to one side, pour in the beaten eggs and scramble for about 1 minute until just set. Break up the eggs into bite-sized pieces.
Add the drained noodles to the pan and pour over the sauce. Use tongs to toss everything together, and stir-fry for 3–5 minutes until the noodles are tender and have absorbed most of the sauce.
Return the chicken and prawns to the pan. Stir in half the bean sprouts and half the chopped chives. Stir-fry for 1–2 minutes, until the sprouts have softened slightly but still retain some crunch.
Divide the Pad Thai between four plates. Garnish with the remaining bean sprouts, chopped peanuts and chives. Serve immediately with lime wedges and extra Sriracha or chilli flakes on the side, if you like.