In a saucepan, combine the grated jaggery and coconut milk.
Heat over low heat, stirring constantly, until the jaggery is completely dissolved.
Remove from heat and let the mixture cool to room temperature.
In a large mixing bowl, beat the eggs until well combined.
Gradually add the cooled jaggery and coconut milk mixture to the beaten eggs, stirring continuously.
Add the ground cardamom, ground nutmeg, ground cloves, vanilla extract, and a pinch of salt. Mix well to combine.
Strain the mixture through a fine sieve to remove any lumps or impurities.
Pour the mixture into a heatproof dish or individual ramekins.
If using, sprinkle the chopped cashew nuts on top of the mixture.
Place the dish or ramekins in a steamer or a large pot with a steamer rack.
Cover and steam over medium heat for about 45-60 minutes, or until the custard is set. To check if it is done, insert a toothpick or skewer into the center; it should come out clean.
Remove the watalappan from the steamer and let it cool to room temperature.
Refrigerate for at least 2 hours before serving.
Cut into slices or unmold the individual ramekins.
Enjoy your meal!