You need more than 1cm because the cauliflower chunks need to be able to bob about a bit. If you've got a fairly solid wok pour in about 3cm oil and heat it until a cube of bread sizzles, floats to the top and browns in fairly quick succession. At that point, add your cauliflower tempura a few at a time. Gently move them about and turn over with a mesh scoop. The only time it gets dangerous is if the oil gets so hot that it starts smoking or if you splash any over the sides. Oil with a high flashpoint like groundnut is better than olive oil for deep frying.