Wash pearl barley well and drain.
Transfer to a pressure cooker pan, add 2 cups water and pressure cook for 4 to 5 whistles or on medium low for 15 minutes after first whistle in a stove top pressure cooker.
If using the INSTANT POT, add 2 cups water and pressure cook on high with seal for 15 minutes and then NPR (naturally pressure release).
Once the pressure drops naturally, open the pan, add jaggery powder, starting with 3/4 cup and mix well. Do feel free to add more jaggery as needed.
Bring to a boil on high (or saute button in Instant Pot) and then simmer on medium for a couple of minutes, stirring often.
Reduce the heat to low and stir in coconut milk and cardamom powder.
Bring to a simmer again on medium low and boil for 5 minutes. After adding coconut milk, try not to cook on high heat; the milk tends to split sometimes.
When using the Instant Pot, feel free to cancel the saute button for a few minutes if you feel the pot is too hot while adding the coconut milk. Turn it on after you have stirred in the coconut milk thoroughly.
Do a taste test and adjust the amount of jaggery and add more coconut milk and/or water to get the right consistency. This payasam thickens as it cools.
To make it more luscious, garnish with ghee or oil roasted (if vegan) chopped cashew nuts and raisins.