1. If using dried chiles, place them in a small bowl and cover them with boiling water, to reconstitute them.
2. Heat a small skillet over medium-high heat. Once the pan is hot (placing your palm close to its base will make you pull it back within two seconds), sprinkle in the coriander and cumin seeds. Toast the seeds, shaking the skillet very often to make sure they turn evenly reddish brown and smell aromatic, about one minute. Transfer the seeds to a blender jar.
3. Drain the soaking chiles and pull out the stems, or cut the stems off the fresh chiles. Add the chiles to the spices in the blender. Pour in the coconut milk, followed by the cilantro, lemongrass, ginger, lime leaves or rind, peanut butter, fish sauce, and salt. Puree these aromatic ingredients to a fluid paste, scraping down the inside of the jar as needed.
4. Transfer the sauce to a small saucepan and bring it to a gentle simmer over medium heat. Cook, uncovered, stirring occasionally, until it starts to thicken a bit, about five minutes. Serve warm.
Any leftover sauce will keep, covered, in the refrigerator for up to one week and in the freezer for up to two months. Thaw and reheat it in a microwave or on the stovetop over low heat.