Place the Szechuan peppercorns in a dry wok and stir over a medium heat until they begin to pop and release their aroma. Transfer to a pestle and mortar and pound with the salt until coarsely ground. Set aside.
Drain the tofu and pat it dry with kitchen paper before cutting it into 20 equal-sized cubes. Pour enough oil into the wok to deep-fry the tofu, and heat it to 190°C (375°F), or until a cube of bread dropped into the oil turns golden in 20 seconds. Add half the tofu and deep-fry until puffy and golden, then remove using a slotted spoon and drain on kitchen paper. Stir-fry the remaining tofu in the same way and set aside. Drain all the oil from the wok and wipe clean with kitchen paper.
Heat the groundnut oil in a wok over a high heat until the oil starts to shimmer. Add the garlic and ginger and stir until golden. Tip in the pork and stir-fry for about 2 minutes until opaque but not golden. Add the prepared ground salt and peppercorns, the rice wine, sugar, oyster and soy sauces, sesame oil, malt vinegar, chilli oil, stock and leeks. Gently slide in the tofu, keeping the pieces separate in the wok, and reduce the heat to a gentle simmer. Heat through for 3 minutes and serve with a scattering of sliced chilli.