Soak the tamarind in 1/2 cup of warm water for about 15 minutes. Rub well you’re your fingers and extract the tamarind pulp. Keep aside. Make a small cut on the skin of two of the plums. Boil them in about 3/4 cup of water. This will take about 5 minutes because they cook very quickly. Do not the discard the water in which the plums were boiled. Let them cool. Peel the skin and discard with the stone.
Smoosh the pulp of the cooked plums with your fingers and strain into a saucepan. Discard the fibre. Add the pink coloured water in which the plums were boiled, to the saucepan. Also add the tamarind pulp, turmeric, salt and curry leaves.
Place the saucepan on the stove and bring to a boil, then turn the heat to medium. Let the tamarind-plum liquid simmer for about 10 minutes till the raw smell of tamarind disappears.
Cut the remaining two plums into quarters and discard the stones. Add them, the mushy well-cooked dal and add another 1/4 cup of water to the saucepan. If the Rasam look a little thick add a little more water. Rasam should have the consistency of a thin soup.
Simmer for about 5 to 7 minutes till plums are soft. Add the rasam powder and powdered jaggery. Taste and adjust to taste. You should have a good balance of somewhat spicy, slightly sour, salty and slight sweetness. Do not cook the Rasam for more than 3 to 5 minutes after adding the Rasam powder.
Stir in the chopped coriander leaves and take the saucepan off the heat. Heat the ghee in a tempering pan. Add the mustard seeds. When they crackle, mix in the asafoetida powder. Take it off the heat and pour into the Rasam.
Serve hot with rice, a dry vegetable curry and papads.