Prepare the dried long red chillies by cutting off the stalks, slicing them lengthways, scraping out the seeds, then soaking them in enough water to cover them for around 15 minutes until soft.
While the chillies are soaking, in a dry frying pan or skillet, toast the coriander seeds and cumin seeds separately until fragrant, shaking the pan often to prevent the seeds from burning. Transfer the spices to a mortar and pestle and pound them finely, then set them aside.
Drain the soaked chillies to squeeze out as much liquid as possible (wash your hands thoroughly with soap afterwards), then roughly chop them.
Using the mortar and pestle, pound the chopped chillies with a ¼ tsp salt, then gradually add the following ingredients, one at a time, pounding each ingredient into a fine paste, before adding the next ingredient, starting with the chopped fresh lemongrass, then chopped galangal, then kaffir lime zest, chopped purple shallots, garlic, and coriander roots.
Once you have finished pounding the paste, stir in the dried spices.
In a small pan over medium, heat 1 cup of the coconut cream and simmer for a couple of minutes, then add the paste, stir it in to combine it well with the coconut cream and fry it gently for 4-5 minutes until fragrant and oily, stirring regularly.
Add the palm sugar, stir it in, then moisten the sauce with the remaining half cup of coconut cream, simmer for a few minutes, then stir in the ground peanuts, simmer for five minutes, then moisten with ½ cup of coconut milk (or equivalent of stock or water).
Add the fish sauce (add a quarter or half first then taste, then add more as you like), chilli powder and optional salt, taste and adjust further if needed to suit your palate. It should be a little oily, rich, dark, sweet, nutty, and spicy.
Remove the satay sauce from the heat and set it aside for an hour or half an hour if you have time – “it will improvely happily”, as David Thompson says. Serve it at room temperature – or warm it up just a little – with your satay skewers.