Remove the stamens from the centre of the zucchini flowers, leaving the flowers intact and being careful not to tear the petals.
Cut the mozzarella into 1¼ in (3 cm) long strips. If using anchovies, cut each in half. Place one piece of mozzarella, one piece of anchovy and one basil leaf, if using, inside each flower, closing the end by gently twisting the petals together.
Prepare the batter by mixing the flour, cornflour, baking powder and sparkling water in a bowl and whisking until smooth and lump-free.
Fill a heavy-based saucepan with vegetable oil and heat until very hot – about 400°F (200°C) on a cooking thermometer. Dip the stuffed flowers in the batter, shake off the excess, then drop them one or two at a time into the hot oil. Cook for 3 to 4 minutes, or until the batter is golden and crunchy, moving them around regularly with a slotted spoon to ensure an even colour.
Drain on paper towel and serve hot.