Place spice paste ingredients in a blender and process till fine.
Place the oil in a pot and saute spice paste with lemongrass stalk and chilli paste till aromatic.
Add the peanuts, stirring, then water, brown sugar, salt and sugar to taste. Cook until mixture is thick, then add the tamarind paste and santan. Simmer for 10 minutes. If necessary, add hot water to adjust texture. Remove from heat and serve immediately with nasi impit.