Cook the soaked black chickpeas with salt till done. Drain and set aside.
Lightly peel the raw bananas - keep some skin on. Chop into cubes. Wash and drain.
Remove the skin of the yam and chop into cubes. Wash well and drain.
Combine raw banana, yam, green chilli, turmeric powder, red chilli powder and salt in a pressure cooker. Add 3/4 cup water and cook till done. The vegetables should be well cooked and really soft.
Add the drained chickpeas, jaggery and black pepper and mix. Add water if needed. Keep on simmer for around 15 minutes, stirring occasionally. Adjust seasoning.
Grind the coconut and cumin to a coarse mixture. Add into the kottu curry and mix. Cook for another five minutes. Switch off.
In a fry pan, heat the coconut oil. Splutter the mustard seeds.
Fry the red chillies, curry leaves and coconut till the coconut turns golden brown.
Pour onto the top and close the lid. Mix at the time of serving.