Heat oven to 160C/140C fan/gas 3. Heat the oil in a flameproof casserole dish over a medium-high heat. In batches, brown the pork belly well, then transfer to a plate. Tip the sugar into the dish and cook over a medium heat until starting to dissolve and caramelise, then quickly stir in the garlic, ginger and star anise, and cook for 1 min.
Carefully pour in the rice wine and soy sauce – watch out, it will spatter – and simmer to dissolve the sugar. Stir the pork into the caramel to coat, then add 100ml water and the five-spice, and bring to a simmer. Put the lid on and cook in the oven for 1 hr 30 mins. Remove from the oven, uncover, put on a high heat to simmer and reduce the sauce until sticky. Can be prepared up to 2 days ahead and chilled. Reheat with a splash of water.
To make the chilli peanuts, mash most of the peanuts using a pestle and mortar, then add the rest of the nuts and roughly crush for a chunky texture. Stir through the togarashi powder. Can be made 2 days ahead and stored in an airtight container.
For the buns, tip the dry ingredients and a large pinch of salt into a food mixer fitted with a dough hook. Pour in the milk, oil and 100ml tepid water, and work the mixture for about 10 mins until smooth and elastic. Transfer to a greased bowl, cover with cling film and leave to double in size (for about 1 hr)
Tip the dough onto a floured surface and roll into a sausage shape. Cut the sausage into 10 equal portions. Roll each portion into a bun, then use a rolling pin to roll each bun out into an oval. Lightly grease each one, then fold them over a greased chopstick or skewer and place on a lightly oiled baking tray to rest for 1 hr or until doubled in size. Remove the chopstick or skewer before steaming.
Heat a steamer and steam the buns on circles of baking parchment in batches for about 10 mins until puffed up. Split the buns and stuff each one with a slice or two of the pork, drizzled with some of the sauce. Scatter over some coriander, cucumber and spring onions, then sprinkle with the crushed nuts and tuck in.