Peel the nendran bananas, slice them and grind to fine paste using mixer grinder. Do not add any water. For ease of grinding I have sliced them, you can grind whole banana directly too.
Heat a heavy bottom wok/pan adding two tea spoon of ghee and roast the broken cashews till they turn little brown. Transfer them and reserve few for garnishing.
Pour the ground banana paste in to same wok along with some ghee and keep stirring in low flame.
When the paste starts thickening, combine sugar, cardamom powder and cashews with it. Add ghee every now and then and continue stirring. The halwa absorbs ghee and halwa texture changes little glossy.
At one stage the halwa leaves ghee that it has absorbed but you have to continue with stirring for about 10 more minutes. By now the color of the halwa would have changed to dark maroon. Switch off the flame.
Transfer the ready halwa on to greased tray and use flat ladle to level the surface. Let it cool.
Using sharp knife mark square halwa pieces and separate them carefully.
Nendran banana halwa is ready and relish it with your family on Diwali.