Peel and cut the sweet potatoes into cubes. Transfer them to a bowl and cover them with water to prevent them from blackening.
If you are using rice, rinse it and soak it in water for about 30 minutes. Then, drain it and grind it into a coarse powder.
Cooking the Kheer:
In a thick-bottomed pan, cook the ground rice with ½ cups of water on low flame. Stir continuously to prevent lumps from forming.
When the rice is half-done, drain and add the sweet potato cubes. Add the thin coconut milk and mix well.
Cover and cook on a low flame until the sweet potatoes become soft and tender.
Once the sweet potatoes are cooked, add the thick coconut milk, cardamom powder, sugar, and a pinch of salt.
Stir everything together and let the kheer simmer for 2 minutes.
Turn off the heat, cover the pan, and let the kheer sit for a few minutes to allow the flavours to blend.
Serve warm or chilled, as you like.
Notes
I will replace the rice with sago or sabudana on a fasting day. Soaking them for about an hour as soaking reduces cooking time. Cook them in the thin coconut milk with the sweet potato.