Mix the cornflour with 2 tbsp water in a bowl and set aside. Heat a wok over medium heat. Once hot, add the oil, then mix in the garlic, ginger, chilli, Sichuan peppercorns and dried chillis. Fry for a few seconds before stirring in the chilli bean sauce, hoisin and ketchup. Once mixed in, pour in the soy sauce, vinegar and stock. Bring to the boil then drizzle in the cornflour mixture and stir well. Remove from the heat and pour into a heatproof bowl. Leave to cool then refrigerate until ready to use. Will keep chilled for up to three days.