First make the dressing. Combine the vinegar, measurement water and sugar in a bowl and stir until the sugar has dissolved. Bring to the boil in a saucepan, then reduce the heat and simmer for 5–6 minutes until reduced by half and syrup like. Leave to cool for 30 minutes, then stir in the soy sauce, chilli and sesame oil. Pour into a serving bowl.
Heat 5 cm (2 inches) vegetable oil in a deep, heavy-based saucepan until it reaches 180–190°C (350–375°F), or until a cube of bread browns in 30 seconds. Meanwhile, cut the tofu into 3 cm (1¼ inch) cubes. Combine the cornflour, salt and Chinese five-spice powder in a bowl. Dip the tofu cubes, a few pieces at a time, in the cornflower mixture, add to the hot oil and deep-fry for 2–3 minutes until crisp and golden. Remove with a slotted spoon and drain on kitchen paper.
Arrange the tofu on a serving platter and dust with a little extra salt and Chinese five-spice powder. Serve with the chilli vinegar dressing for dipping.