Set aside a flat bottomed plate lubricated with ghee. Dry roast the cashew nuts in a large Kadai over low flame. It can be roasted without darkening. Allow cooling after expelling from heat. Slog it to fine and coarse granules. Do not crush for too quite a while now as this induces oil to be cleared.
In a Kadai, heat the jaggery and water around each other. Add a subtle ball of jaggery syrup. Keep a bowl of water close to check the continuity. Continue to stir the syrup and test the stability by placing a drop of syrup in water. If it completely dissolves, you should keep simmering. If the syrup starts to settle at the bottom, you could indeed wrap it into a tiny ball with your palms.
On low heat, mix the cashew powder and cardamom powder with the sugar syrup. To prevent lumps, stir it often. Cooking time is around 10-15 mins. The barfi is done once it starts to depart the edges. To test the reliability, carry a few of the combinations in a ladle and wrap it in a softball.
Pour the mixture into the lubricated plate and spread it evenly out. Once it is lukewarm, use a knife to slice it into diamond shapes. It should be protected from light. Serve immediately.