Make your tempura batter by quickly whisking ice-cold sparkling water into the tempura flour. Add the lemon zest and season with salt and pepper but do not over stir the batter as gluten will start to form.
Deep fry the popcorn calamari at 180 degrees Celsius / 360 Fahrenheit. If you do not have a deep fat fryer it is fine to use a deep saucepan, just make sure to use a thermometer to ensure you have the right temperature.
Use a pair of tweezers or small metal cooking tongs to pick the popcorn calamari out of the bowl of batter to ensure they separate from one another and do not end up as one enormous clump.
Add the battered calamari popcorn to the hot cooking oil carefully and keep an eye on the oil temperature. Adding it all at once seems tempting but this can bring down the temperature too much.