Crush the peanuts with mortar and pestle or use a food processor to ground the peanuts. Set aside.
In a small bowl, add the tamarind pulps plus 1/4 cup warm water. Set aside for 15 mins. Squeeze and extract the juice from the tamarind pulps and discard. Keep the tamarind juice.
Chop the Spice Paste ingredients coarsely, transfer to a food processor and blend until very fine. Add a few tablespoons of water to help blending.
In a sauce pan, heat the oil on medium heat and add the spice paste.
Add the remaining two strips of lemongrass to the spice paste. Cook the spice paste until it becomes aromatic and smell spicy.
Add the ground peanuts, water, tamarind juice, salt, sugar, coriander powder and sweet soy sauce. Stir to combine well.
Turn the heat to medium-low heat, stir continuously for about 5-10 minutes or until the peanut sauce thickens to your desired consistency. The oil and the peanut sauce should separate when it's done.
Let cool at room temperature and serve the peanut sauce with satay.