Soak the vadan samba for 2 hours, drain the soaking water. Now bring 7 cups of water to boil in a heavy-bottomed vessel. Once the water is simmering, add the rice and bring to a boil. Lower the heat. Cover partially, not fully, or the rice will boil over. The rice should be well cooked in about an hour.
Now you can either leave this rice in its cooking water overnight to sour slightly, or you can drain out the cooking water and use that as the base for the kanji. If you’re leaving it out to sour, make sure the rice is well covered in water. Add more if needed.
For fresh kanji: drain the rice and save the starchy cooking water—this is kanji.
For rice left overnight: skim the gelatinous layer that will likely have formed on top and reserve. Now drain the rice and add that to the skimmed gelatinous layer.
Fermented or not, take a few tablespoons of the rice, mash it well (or use a blender) and add that back into the kanji. You’ll want about 4-5 cups of mashed rice plus cooking water at this stage, so adjust accordingly.
Place the kanji in a saucepan over a gentle simmer. Add in the karupatti/ panavellam, and allow this to dissolve. Check the texture – if too thin, you can add more mashed rice. If too thick, add a little water. Keep in mind that coconut milk thins the whole kanji later, so you want it thickish, not too watery at this stage. Heat through and turn off the flame.
Separately, grind the spices. Sitharathai and sukku can be tough to hand pound, so it’s best to use a spice or coffee grinder. Pulverize all the spices together, and then use a strainer or sieve to remove the fibrous parts. Add the spice powder to the kanji, along with salt.
Taste, and adjust sweetness if you wish. The spice tastes should be strong, but not overpowering. They’ll get further mellowed with the addition of coconut milk.
You can store the kanji in this form for a few days, refrigerated. You’ll need to thin it slightly and warm before serving.