Your fizzy water needs to be icy cold, so either chill it in a container in the refrigerator overnight, or pop a container in freezer while you get organised.
Fill your deep fat fryer (or a suitably large and high-sided saucepan) to one third full with oil. It's important not to overfill your fryer as oil expands when heated and can bubble up further when food is added. Heat to 180C (355F). Keep your oil supervised at all times.
Get your zucchini / courgettes sliced into fries so that they're on hand to use as soon as your batter is ready.
Put the flour, cornflour and a pinch of salt in a bowl and mix together.
Add the egg yolk and chilled fizzy water and whisk until you have a smooth batter. Whisking in one direction only can help avoid lumps.
Drop a few pieces of courgette into the batter and then straight into the oil. Don’t drop all the veggies into the batter at once as they can change the temperature and thickness of the batter if they sit in it too long. You also need to work in batches so that you don’t overcrowd the frying pan.
Fry for 3-4 minutes until very crispy and puffed, turning gently as needed. They shouldn't take on much colour.
Lift from the pan using a slotted spoon or straining spoon. Place on a plate lined with kitchen paper to allow the oil to drain.
If you like, you can place the drained tempura on a baking sheet in a low over to stay hot and crisp while you fry the rest of the batch.