I used to have real problems making a good tempura batter, but for years, I have used a recipe and method from a Delia Smith book here in the UK. It has proved itself time and time again, so although the recipe is written for beginners, and therefore may sound patronising to an experienced cook, it WORKS!
2 oz (50 g) cornflour
1 large egg
3 fl oz (75ml) ICED water
The first thing you need to do is weigh out the cornflour, put it in a polythene bag and then place it with the unbroken egg in the fridge for about 1 hour to get thoroughly cold. At the same time place a few ice cubes and about 3 fl oz (75 ml) cold water in a bowl or jug and put that in the fridge as well.
When you're ready to cook the tempura, begin to heat the frying oil, using a thermometer to check how hot it is getting. Have a plate with absorbent kitchen paper ready for draining and keep it in a warm place. When the oil reaches 175°F (90°C), take the chilled ingredients out of the fridge.
In a bowl using chopsticks, blend the egg with 3 fl oz (75 ml) of the iced water, not too thoroughly – it's more blending than whisking. Now sift the cornflour, and what's going to be really hard now is to resist the temptation to whisk, but instead very lightly blend leaving some free cornflour unblended, and totally ignoring the lumps!
When the oil reaches 350°F (180°C), measure 1 heaped tablespoon of cornflour on to a plate – use this to coat the ingredients (eg prawns, thinly sliced aubergine etc) before they go into the batter. After coating with the cornflour, hold the tail of the prawn (as an example) with cooking tongs, then quickly coat it with batter, and if this includes a lump, all the better. Now keep hold of the tail, lower the prawn into the hot oil and then release the tail about 30 seconds later. Now do the same with another prawn and when they float to the top and the batter turns golden, they're cooked, so now remove them to the plate lined with kitchen paper to drain.