Combine the chiles, chiltepins (if using), onions, garlic, fish paste, and l/2 cup peanut oil in a food processor and puree to a smooth paste. Heat the 1/4 cup peanut oil in a wok or skillet to high heat, add the paste, and fry until it is dark in color and the oil starts to separate. Add the tamarind water, salt, and brown sugar and simmer for 5 minutes, stirring occasionally. Store in bottles in the refrigerator for up to a week.
