Heat the oil in a large pot with a heavy bottom, or in a deep fryer to 350 degrees F. Make sure the oil level is no more than one-third of the pot as it will rise during the frying process.
Mix the cardamom, paprika and salt together in a small bowl.
Cut the sweet potatoes into 1/4-inch-thick slices, and then cut the slices into 1/4-inch sticks. Put the cornstarch into a large bowl and add the sweet potatoes, tossing well using your hands to make sure they are well coated.
Working in batches, fry in the hot oil until golden brown, 6 to 7 minutes. Drain on paper towels, and then sprinkle liberally with the spice mixture. Serve with Chili Coconut Dip.
Chili Coconut Dip:
Heat the oil in a skillet over medium heat and add the curry leaves, coconut, chili powder, ground coriander, mustard seeds, salt and pepper. Toast until the coconut is golden brown, 2 to 3 minutes. Place in a bowl with the mayonnaise and stir well to combine.