Soak the rice in water for 4 to 5 hours. Drain the water, grind it along with coconut, cooked rice and mango puree until you get a smooth batter. Add the sugar and salt, grind again. Pour the batter in a vessel, cover it. Allow it to ferment 1 whole night.
Make the appam:
The next day, just before making your appam, add the soda by mixing it in 1 tbsp of water. Mix it well.
Grease a stainless steel bowl with little ghee.
Mix the batter with some water if it's too thick. Pour it in the bowl, sprinkle with cardamom powder.
Steam it in a steamer or pressure cooker for 30 minutes. To check if it's cooked, insert a skewer and if it comes out clean, then the appams are cooked. Garnish with crushed nuts.