Fry the peanuts to bring out their toasty flavour. Heat the oil in a wok or frying pan over a high heat. When shimmering, add the nuts and stir-fry for a minute. Reduce the heat to medium and continue to fry, stirring often, for about 5 minutes, until the nuts are deep golden brown. Remove with a slotted spoon and drain on paper towel.
Tip the nuts into a food processor (keep the skins on) with the garlic, shrimp paste, palm sugar, and a good pinch of salt. Blitz to a sandy rubble, then scrape the sides and process for another minute. Continue scraping and blitzing for a few minutes at a time until you have a smooth purée (it should get there, with some perseverance).
Transfer the peanut butter to a saucepan, add the kecap manis and a small glassful of water. Cook over a medium-high heat, stirring often, until you have. a sauce that is dark and slightly sticky. Thin with more water if it gets too thick- you want it to thickly coat the sate but be just pourable. Leave to cool and taste for seasoning, adjusting the saltiness and sweetness if you like. A final spritz of lime juice will brighten the flavours.