Heat the sesame oil and roast the whole spices in it, along with the curry leaves. Let them cool down and grind to a fine paste with a little water added to it.
If you are making tamarind extract, soak tamarind in 1 cup of hot water and squeeze out the pulp from it once the water has cooled down a little. Sieve the pulp to get rid of any seeds or other thick tamarind skin. Repeat with 1 more cup of room temperature water.
If using tamarind paste, for 1 key lime size ball of tamarind, use 1 ½ teaspoon of tamarind paste and mix it in about 2 cups of water.
Add this tamarind water to a saucepan along with 1 more cup of water and bring it to a boil.
Add the spice paste / masala paste to this and mix well. Rinse out the blender jar with ½ cup of water and add this to the saucepan.
Add turmeric powder, jaggery and salt. Mix well and let it boil for about 5-6 mins, till the raw tamarind smell goes away.
If you want to thicken the curry a little, reduce the heat to medium and let it cook away for another 10 mins or so.
Turn off the heat. Heat some more sesame oil / nallennai in the pan used to roast the spices.
Add mustard seeds to the hot oil. Once they splutter, add the sundakkai and let it start to fluff up a little and start to turn color. Don't let it burn. Add some curry leaves as well and pour this into the saucepan with the tamarind-spice mix.
Give it a quick mix. You can add some chopped cilantro as garnish as well.