Place the aubergine rounds on a sheet pan lined with paper towels, and sprinkle with salt. Cover the aubergine with paper towels, and place a heavy skillet over it to press out the excess moisture. Set the pan aside for 30 minutes, and then quickly rinse the aubergine rounds, and pat dry.
Place the gram flour, rice flour, chile powder, ground cumin, ground coriander, and mango powder into a bowl. Season the mixture with salt, to taste.
Gradually add the cold water in a thin stream, whisking continuously to form a batter with the consistency of single cream.
In a large heavy-bottomed saucepan, pour enough oil to fill the pan about a third of the way. Heat over medium heat until a deep-frying thermometer inserted in the oil reaches 360 degrees F or until a breadcrumb sizzles and turns brown when dropped into it.
Dip the aubergine rounds into the batter one by one and carefully lower them into the hot oil in batches. Fry for 2 to 3 minutes on each side, or until crisp and golden-brown on both sides, and completely cooked through. Remove the aubergine rounds from the oil with a slotted spoon, and set aside to drain on a sheet pan lined with paper towels. Keep warm on a hot plate. Repeat the process with the remaining aubergines.
Heat 1 tablespoon of the oil in a separate frying pan over a medium heat. When the oil is hot, add the curry leaves, if using, and fry for 1 to 2 minutes, or until crisp. Remove the leaves from the pan with a slotted spoon, and set aside on a sheet pan lined with paper towels, and then sprinkle with a pinch of salt.
To serve, divide the spiced aubergines among six serving plates and garnish with the curry leaves.
*Can be found at specialty Asian markets. **Can be found at specialty Indian markets. Once the aubergine is soft all the way through, it is done. Serve this dish as soon as possible, as the batter will soften otherwise. This makes a great snack with drinks - dip the aubergines in a coconut or coriander chutney.