One of my favourite breakfasts in Taiwan is xian dou jiang - hot, savoury soy milk, lightly curdled with vinegar and flavoured with preserved vegetable and tiny shrimp. It's very easy to make, and goes deliciously with you tiao (Chinese fried doughnut), to dunk in the soybean milk. You tiao come in pairs, and can be heated in a 180°C oven (350°F) for about eight minutes if they're stale.
The savoury soy milk is also good with shao bing (flaky sesame bread) that's been split in half lengthwise and stuffed with scrambled egg and more you tiao, but that requires quite a bit of work.
The quality of the soybean milk is very important - if possible, buy it from a shop that makes their own, rather than mass-produced stuff. Check the label to make sure it doesn't contain sugar.
The dried shrimp should be tiny and delicate - don't use the hard dried shrimp that need to be soaked before it's used. Look for the tiny shrimp in shops selling Chinese, Japanese or Thai products, and if possible, sniff them before buying, to make sure they don't smell of ammonia.
Briefly rinse the shrimp under running water. Drain and put them in a saucepan with the soy milk. Add a pinch of salt and the soy sauce, then heat until simmering. Taste the mixture and add more salt, if needed.
While the soy milk is heating, prepare the other ingredients. Finely chop the preserved mustard stem (zha choi) and mince the spring onion. Split the you tiao (Chinese fried doughnut) in half lengthways (they come in pairs), then cut into 5mm (¼ in) slices.
Divide the preserved mustard stem between two bowls, top with the hot soy milk and add half a teaspoon of rice vinegar to each portion. Stir once, then leave for a couple of minutes to curdle slightly.
Add several pieces of you tiao to each bowl and sprinkle with the spring onion. Drizzle with chilli oil, then serve immediately, with more you tiao on the side.