Gently open each courgette flower, remove the pistil and stamens, and wipe the petals inside and out with a damp tea towel; set aside.
In a small bowl, combine the ricotta, grated Parmesan, chopped chives, chopped parsley, 1/4 tsp salt and a pinch of black pepper; stir until smooth.
Spoon about 1-2 tsp of the ricotta mixture into each flower, then gently twist the petals to seal the filling.
In a medium bowl, whisk together the plain flour, cornflour, baking powder, 1/2 tsp salt and 1/4 tsp black pepper; gradually pour in the very cold sparkling water, whisking until just combined (the batter should still be slightly lumpy).
Pour the oil into a large, heavy-based saucepan to a depth of about 5cm and heat over medium-high heat until it reaches 175-190°C, or until a small drop of batter sizzles and rises immediately to the surface.
Working in batches of 3-4, dip each flower into the batter, letting any excess drip off, then carefully lower into the hot oil; fry for 2-3 minutes on each side until golden and crisp.
Transfer the fritters to a wire rack set over kitchen paper, then immediately sprinkle with extra flaky sea salt; serve hot with lemon wedges.