Sift the gram flour, baking powder, turmeric, coriander, cumin and chilli powder into a large mixing bowl. Season with salt and gradually add the soda water to make a thick batter, being careful not to overmix.
Pour sunflower oil into a wok until one-third full and heat to 180–190°C (350–375°F), or until a cube of bread browns in 30 seconds. Dip the courgette batons in the spiced batter and then deep-fry in batches for 1–2 minutes or until crisp and golden. Remove with a slotted spoon and drain on kitchen paper. Serve the courgettes immediately with thick natural yogurt, to dip.