I like this for the contrast in textures: the slippery, tender tendon and the firmer pieces of bean curd. If you dislike tendon, use a total of about 750 grams of bean curd.
Bring a medium-sized pot of water to the boil, add the tendon and let the water boil again. Strain the tendon through a colander and rinse it thoroughly. Wash the pot, then fill it halfway with water and bring to the boil. Add the tendon, bring to the boil then lower the heat and simmer for about two hours, or until the tendon is very tender. If you have a pressure cooker, blanch and rinse the tendon, then put it in the pot that is about half full of water. Cook on high pressure for about one hour, then remove from the heat and let the cooker cool naturally before opening the lid. Cut the tendon into bite-sized pieces.
Briefly rinse the black beans in a small sieve, then put them in a bowl and add 15ml (1tbsp) of warm water and leave to soak for about 15 minutes. Put the Sichuan peppercorns in a small, unoiled skillet placed over a medium flame. Cook, stirring frequently, until the peppercorns are toasted. Cool the peppercorns, then remove the shiny seeds. Grind the husks to a rough powder in a mortar.
Heat a wok over a high flame, then add about 10ml (2tsp) of oil. When the oil is hot, add the doubanjiang and stir briefly. Mix in the black beans and the soaking liquid, Sichuan peppercorns and chilli flakes, then add the sesame oil, chilli oil and chicken stock. Stir well, then bring to the boil and taste the liquid; season with salt and more doubanjiang, chilli flakes and/or chilli oil, if needed. Mix in the beef tendon pieces and simmer for several minutes.
While the tendon is simmering, cut the bean curd into 2.5cm (1in) squares. Trim off and discard the root ends of the leeks, then slice them about 1cm (7/16 inch) thick, using the white and pale green parts only. Put the cornstarch into a bowl and mix in about 30ml (2tbsp) of water. Cut the spring onions about 3mm (⅛in) thick.
Add the bean curd and leeks to the wok and stir them in. Simmer for a few minutes, or until the leeks start to soften. Taste again for seasoning and correct, if needed.
Stir the cornstarch and water together, then drizzle about half of this mixture into the ingredients in the wok. Mix well and check the consistency of the liquid in the wok – it should be a glossy sauce that very lightly coats the other ingredients. If needed, stir in a little more cornstarch/water. Ladle the mapo tofu into a serving bowl, sprinkle with spring onions, then serve with steamed white rice.