Wash sweet potatoes and dice them into 1/2-inch cubes. You don't have to peel the skins.
Put in a microwave-safe bowl with 2 tbsp water and cover with a microwave-safe lid (or just a ceramic plate). Microwave for 5-7 mins until tender (if you prefer, you can also cook the sweet potatoes on the stovetop).
Heat a skillet and dry-roast the red chillies, coriander seeds, cumin seeds until they are a couple of shades darker and smell aromatic.
Put the roasted spices in a blender along with the coconut milk. Add the coconut milk, tamarind and enough water to blend into a smooth paste.
Pour the coconut paste in a saucepan and add the sweet potatoes. Bring the mixture to a boil. If it's too thick, add more water.
Add salt to taste and let the curry simmer for about 10 mins. Turn off the heat and garnish with coriander leaves.