Special equipment: spice grinder or coffee grinder steamer or steamer basket, cheesecloth or paper towel, string hopper press, 12 thatu, see Cook's Note**
In a large stainless steel pot combine the coconut milk and 3 cups water. Whisk in the tamarind pulp. Over high heat, bring the mixture to a boil, then remove from the heat. Add the saffron and steep for 15 minutes. Strain the mixture and reserve liquid for later.
Melt the unsalted butter in a large pot over medium heat. Add the onions and saute for 5 minutes until soft and just beginning to brown. Add the Thai chiles, Sambhar Spice Mix, ground coriander, ground cumin, curry powder, and turmeric powder. Stir to combine, reduce heat to low and simmer for 3 to 4 minutes to bring out the flavor of the spices. Add the remaining 4 cups water, channa dal, potatoes and tomatoes and cook until tender, but not mushy, about 18 to 20 minutes.
Add the reserved coconut/tamarind liquid and simmer for 5 minutes. Serve warm and garnish with the cilantro leaves.
In a heavy skillet, over very low heat, dry roast the channa and urad dahl, stiring constantly, for 5 to 8 minutes. See Cook's Note**. Remove the dal from skillet and set aside to cool.
Increase the heat to medium high, add the coriander seeds, cumin seeds, black peppercorns, fenugreek seeds and dried chilies to the skillet, stirring constantly. Toast the spices until they release their aroma, about 3 minutes. Grind the dal to a powder in a spice grinder or coffee grinder. Grind the toasted spices to a powder. Combine the dal, ground spices and turmeric powder. Store in a tightly sealed jar.
Line a steamer with several layers of cheesecloth and fill with the all-purpose flour. Steam for 45 minutes. Spread the steamed flour on a baking sheet to cool. Combine the steamed flour, rice flour, and salt in a large bowl.
Make a well in the center of the flour and add the hot water. Mix with a spoon until you can gather the dough together into a ball. On a clean surface, knead the dough lightly for 2 to 3 minutes Press the dough through the hopper press onto the thatu. Place 6 filled thatu in the steamer and steam for 5 to 8 minutes. While they are steaming fill the remaining thatu with dough, then steam.
Channa Dal is a yellow split pea, Urad Dal is a white split gram bean and can be found in specialty Indian stores and health food stores, as well as the spice seeds. Dry Roasting means to roast without adding oil, using a hot, dry skillet. Can be found in some supermarkets and health food stores A string hopper press is a tool used for extruding dough, similar to the way a pasta machine extrudes noodles. It is usually made of wood or metal and can be found in specialty Sri Lankan stores. Thatu are perforated disks made of plastic or woven bamboo. They are the forms the string hoppers are steamed on. Thatu are also available at Sri Lankan stores.