To make the filling, finely slice the garlic and ginger. Pick the coriander leaves and set aside, then finely slice the stalks. Heat a splash of groundnut oil in a large frying pan over a medium-high heat, then add the garlic, ginger and coriander stalks. Fry for around 3 minutes, or until golden. Slice the mushrooms, then add to the pan for around 5 minutes, or until lightly golden brown.
Add the vinegar, chilli sauce and soy, then cook for a further 5 minutes, or until the liquid has boiled and bubbled away and the mushrooms are golden brown and starting to caramelise. Transfer to a bowl.
Trim and finely slice the spring onions, then add the white part to the bowl. Deseed and finely slice the chilli, then add half to the bowl, reserving the rest for later. Stir in the sesame oil, then set aside.
Add the coconut milk, 2 heaped tins’ worth of flour and a good pinch of salt to a food processor. Whiz to a dough, then transfer to a flour-dusted surface and roll into a thick sausage. Cut into 12 equal-sized pieces, roll into balls, then flatten into rounds, roughly ½cm thick.
Equally divide the mushroom mixture between each of the 12 dough circles (you'll need roughly 1 tablespoon of filling per circle), making sure to leave a 2cm gap around the edges. Pull and fold the sides over the filling, pinching the edges together to seal. Place upside-down (so the scruffy edges are underneath) in double-layered, lightly greased muffin cases and divide between two bamboo steamer baskets.
Place a wok over a high heat, then fill with 1 litre of boiling water and pop the steamer baskets on top. Reduce the heat to medium and steam for around 12 minutes, or until piping hot through and puffed up.
Meanwhile, toast the sesame seeds in a small frying pan over a medium heat. Once the buns are ready, sprinkle over the seeds and the reserved spring onions and chilli. Tear the coriander leaves on top, then serve with hoisin sauce for dipping.