Heat 500 ml of full cream milk in a heavy bottom pan.
Bring to a boil and then simmer and cook till it has reduced to half in volume. Make sure to stir in between.
Once the milk is reduced, add 200 ml of condensed milk to it. Mix well.
Cool this mixture completely.
Meanwhile, break open the tender coconuts and extract the coconut water. We will use the coconut water from only one of the coconuts (roughly 1.25 cups of coconut water), however use the coconut flesh of both the coconuts.
Scrape the coconut flesh from both the coconuts.
Finely chop half of the coconut flesh. Keep this aside.
Transfer the remaining coconut flesh in a mixer jar. Add some coconut water (roughly ½ cup) and blend to a fine paste.
Now add some more coconut water (around ¾ cup) and blend again. Tender Coconut puree is ready.
Heat a pan and add 2 teaspoon of ghee and toast the cashews till they are golden brown.
Now to the cooled milk mixture, add the tender coconut puree. Also add around 150 ml of regular coconut milk and mix well.
Add the finely chopped tender coconut flesh, roasted cashews and ½ tsp cardamom powder.
Mix everything well. If the mixture is too thick, you could add some more coconut water to adjust the consistency.
Garnish with some finely chopped pistachio nuts. Refrigerate the payasam for at least 4 to 5 hours before serving.